The removal of arsenic and metals from highly acidic water in horizontal subsurface flow constructed wetlands with alternative supporting media

A comprehensive review on arsenic contamination in groundwater: Sources, detection, mitigation strategies and cost analysis

While groundwater is commonly perceived as safe, the excessive presence of trace metals, particularly arsenic (As), can pose significant health hazards. This review examines the current scenario of pollutants and their mitigations focusing on As contamination in groundwater across multiple nations, with a specific emphasis on the Indian Peninsula. Arsenic pollution surpasses the WHO limit of 10 ppb in 107 countries, impacting around 230 million people worldwide, with a substantial portion in Asia, including 20 states and four union territories in India. Analysis of the correlation between the aquifer and arsenic poisoning highlights severe contamination in groundwater originating from loose sedimentary aquifer strata, particularly in recently formed mountain ranges with geological sources presumed to contribute over 90% of arsenic pollution, i.e. a big environmental challenge. A myriad of techniques, including chromatographic, electrochemical, biological, spectroscopic, and colorimetric methods among others, are available for the detection and removal of arsenic from groundwater. Removal strategies encompass a wide array of approaches such as bioremediation, adsorption, coagulation/flocculation, ion exchange, biological processes, membrane treatment, and oxidation techniques specifically tailored for affected areas. Constructed wetlands help to eliminate heavy metal impurities such as As, Zn, Cd, Cu, Ni, Fe, and Cr. Their efficiency is influenced by design and environmental factors. Nanotechnology and nanoparticles have recently been studied to remove arsenic and toxic metal ions from water. Cost-effective solutions including community-based mitigation initiatives, alongside policy and regulatory frameworks addressing arsenic contamination, are essential considerations.

Waste-derived substrates in vertical-flow constructed wetlands for an efficient removal of high-concentration heavy metals

Contamination by heavy metals (HMs) in aquatic ecosystems is a worldwide issue. Therefore, a feasible solution is crucial for underdeveloped and developing countries. Waste-derived materials (WDMs) exhibit unique physical and chemical properties that promote diverse mechanisms for the removal of HMs in constructed wetlands (CWs). In this study, we aimed to report the removal efficiency of HMs of vertical-flow constructed wetland (VFCW) systems using different WDMs, such as clinker brick (Jhama), eggshells, and date palm fiber (DPF). Synthetic wastewater with high concentrations (3.3-61.8) mg/L of HMs (As, Cr, Cd, Pb, Fe, Zn, Cu, and Ni) was applied to the systems followed by 3 days of hydraulic retention time. The results demonstrate that removal efficiencies of HMs ranged between 94.8 and 98.7% for DPF, 95.4-98.5% for eggshells, and 79.9-92.9% for the Jhama-filled CWs, while the gravel-based systems were capable of 73-87.6% removal. Two macrophytes, Canna indica and Hymenocallis littoralis were planted in the CWs and exhibited significant accumulation of HMs in their roots. The study reports that WDMs are effective for concentrated HM removal in CWs, and macrophytes demonstrate significant phytoremediation capabilities. The findings of this study will facilitate the economically feasible and efficient design of CWs for effectively treating concentrated HMs in wastewater.

Mathematical modeling of subsurface flow constructed wetlands performance for arsenic removal: Review and perspectives

Constructed wetlands are nature-based solutions able to remove different pollutants from water, including arsenic. Arsenic is a pollutant of concern given its toxicity and its presence in water sources worldwide. Despite the increased interest in investigating the performance of constructed wetlands in the treatment of arsenic-contaminated water at the laboratory scale, the application of these solutions at the pilot and full scale is still limited. To understand and predict the removal of arsenic in constructed wetlands, some numerical models have been developed. Among black box models, only first-order models have been proposed, with unsuccessful results. The model that best describes arsenic retention processes in constructed wetlands is RCB-ARSENIC, a mechanistic model adapted from Retraso-CodeBright that simulates arsenic reactive transport. This model includes arsenic precipitation, arsenic sorption on supporting media, arsenic sorption on plants roots and arsenic uptake by plants; represented in the reactive term of the reactive transport equation. Thus, it includes two of the three main processes that remove arsenic in constructed wetlands: precipitation, sorption, and coprecipitation. Despite this, and what is known about arsenic geochemistry, the formulation of these reactive rates requires improvement. In addition, this model was calibrated and validated using data from a single horizontal subsurface flow constructed wetland system, which treated one type of synthetic water. Therefore, it cannot be applied to other types of arsenic-contaminated water or other constructed wetland systems. Moreover, the reactive transport of relevant species -especially iron- and their role in arsenic removal, along with relevant redox reactions associated to the presence of organic matter, oxides or bacteria-, must be included. A comprehensive mechanistic model able to simulate different design, environmental and operation conditions may be used to guide the design of constructed wetlands targeting the removal of arsenic.

Arsenic pollution cycle, toxicity and sustainable remediation technologies: A comprehensive review and bibliometric analysis

Arsenic pollution and its allied impacts on health are widely reported and have gained global attention in the last few decades. Although the natural distribution of arsenic is limited, anthropogenic activities have increased its mobility to distant locations, thereby increasing the number of people affected by arsenic pollution. Arsenic has a complex biogeochemical cycle which has a significant role in pollution. Therefore, this review paper has comprehensively analysed the biogeochemical cycle of arsenic which can dictate the occurrence of arsenic pollution. Considering the toxicity and nature of arsenic, the present work has also analysed the current status of arsenic pollution around the world. It is noted that the south of Asia, West-central Africa, west of Europe and Latin America are major hot spots of arsenic pollution. Bibliometric analysis was performed by using scopus database with specific search for keywords such as arsenic pollution, health hazards to obtain the relevant data. Scopus database was searched for the period of 20 years from year 2003-2023 and total of 1839 articles were finally selected for further analysis using VOS viewer. Bibliometric analysis of arsenic pollution and its health hazards has revealed that arsenic pollution is primarily caused by anthropogenic sources and the key sources of arsenic exposure are drinking water, sea food and agricultural produces. Arsenic pollution was found to be associated with severe health hazards such as cancer and other health issues. Thus considering the severity of the issue, few sustainable remediation technologies such as adsorption using microbes, biological waste material, nanomaterial, constructed wetland, phytoremediation and microorganism bioremediation are proposed for treating arsenic pollution. These approaches are environmentally friendly and highly sustainable, thus making them suitable for the current scenario of environmental crisis.

Immobile Iron-Rich Particles Promote Arsenic Retention and Regulate Arsenic Biotransformation in Treatment Wetlands

Remediation of arsenic (As)-contaminated wastewater by treatment wetlands (TWs) remains a technological challenge due to the low As adsorption capacity of wetland substrates and the release of adsorbed As to pore water. This study investigated the feasibility of using immobile iron-rich particles (IIRP) to promote As retention and to regulate As biotransformation in TWs. Iron-rich particles prepared were immobilized in the interspace of a gravel substrate. TWs with IIRP amendment (IIRP-TWs) achieved a stable As removal efficiency of 63 ± 4% over 300 days, while no As removal or release was observed in TWs without IIRP after 180 days of continuous operation. IIRP amendment provided additional adsorption sites and increased the stability of adsorbed As due to the strong binding affinity between As and Fe oxides. Microbially mediated As(III) oxidation was intensified by iron-rich particles in the anaerobic bottom layer of IIRP-TWs. Myxococcus and Fimbriimonadaceae were identified as As(III) oxidizers. Further, metagenomic binning suggested that these two bacterial taxa may have the capability for anaerobic As(III) oxidation. Overall, this study demonstrated that abiotic and biotic effects of IIRP contribute to As retention in TWs and provided insights into the role of IIRP for the remediation of As contamination.

An innovative reuse of bottom ash from municipal solid waste incinerators as substrates of constructed wetlands

The incineration of municipal solid waste has been important in waste management, but it raises another environmental issue concerning residue treatment. This study describes an innovative use of naturally aged incineration bottom ash (AIBA) as an alternative substrate for horizontal subsurface flow (HSSF) constructed wetlands (CW). Although experimental results from a period lasting for 396 days only revealed slightly higher removal ratios in HSSF with AIBA (HSSF-E) than in HSSF-traditional pebble beds (HSSF-C), increasing from 67% to 76% for BOD, 44%-51% for TKN, 47%-54% for NH₃-N, and 44%-52% for TN. The data indicate that the use of AIBA in HSSF CW can achieves a certain removal efficiency of BOD and nitrogen species. Interestingly, the total phosphorus removal rates also increased significantly from 20% in HSSF-C to 36% in HSSF-E. These observations on the use of AIBA in HSSF CW confirmed that AIBA is a suitable alternative for use as a substrate for HSSF CWs and identified an additional way to reuse incineration bottom ash. Design criteria for a CW using AIBA as a partial substrate is proposed to improve the pollutant removal performance of HSSF CWs.

Enhanced removal of heavy metals and metalloids by constructed wetlands: A review of approaches and mechanisms

Constructed wetlands (CWs) are increasingly employed to remediate heavy metal and metalloid (HMM)-polluted water. However, the disadvantages of HMM removal by conventional CWs (without enhancement), such as an unstable and unpredictable removal efficiency, hinder the reliability of this technology. The objective of this study was to review research on enhanced CWs for HMM removal. In particular, we performed a bibliometric analysis to evaluate research trends, critical literature, and keyword evolution in recent years. Subsequently, we reviewed various enhanced approaches for the application of CWs for the removal of HMMs, including the use of improved substrates, aquatic macrophytes, microorganisms, bioelectrochemical coupling systems, hybrid CW, external additives, and operation parameters. Furthermore, the main mechanisms underlying HMM removal by these approaches are summarized. Our review clearly reveals that research on the remediation of HMM-polluted water via CW technology is receiving increased attention, with no apparent trends in topics. The selection of appropriate enhanced approaches or operation parameters as well as methodological improvements should be based on the dominant environmental conditions of the CW column and removal mechanisms for the targeted HMMs. Based on the established literature, several suggestions are proposed to guide the optimization of the design and operation of efficient CWs for the treatment of HMM-polluted water.

A review on the removal of heavy metals and metalloids by constructed wetlands: bibliometric, removal pathways, and key factors

Heavy metals and metalloids (HMMs) pose a serious threat to both environmental and human health. The unique characteristics and environmental toxicity of HMMs make their removal from the environment a major challenge. Constructed wetlands (CWs) are increasingly being used as an eco-friendly system for the removal of HMMs from aqueous environments. In this review, bibliometric analysis was performed using the Scopus database using VOSviewer software to assess the developing use of CWs in recent years. Heavy metal and metalloid (HMM) removal pathways were reviewed (such as precipitation, co-precipitation, adsorption and ion exchange, plant action and microbial action) along with the impact of key factors (pH, chemical oxygen demand, dissolved oxygen, HMM concentration, and temperature). This review aimed to establish the connections between published results, to help effectively optimize the use of CWs for the removal of HMMs and identify the most critical factors for their effective removal. Important aspects that require further research include assessing the synergistic toxicity between different pollutants and combining the use of CWs with other technologies to optimize pollutant remediation efficiency.

Polyethylenimine-Modified Magnetic Chitosan for the Uptake of Arsenic from Water

The removal of heavy metals from water has become a global environmental problem. Various materials have been applied as adsorbent to remove metals from water. In this field, nanomaterials have been gaining increasing interest due to their exceptional properties. In this work, we discuss the synthesis of a core-shell structure nanocomposite by the modification of magnetic chitosan (CS) (Fe3O4/CS) with polyethylenimine (PEI) to produce Fe3O4/CS/PEI composite for the adsorption of arsenic ions (As(V) and As(III)) from aqueous solution. The synthesized materials were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM). The results indicated the successful combination of three components of the nanocomposite. The adsorption conditions were optimized by studying the effect of different parameters included pH, contact time, initial concentration, and adsorbent dosage. The optimum adsorption pH was found to be 6.7 while the optimum adsorbent dosage was found to be 2.0 and 1.5 g/L for As(III) and As(V), respectively. The removal efficiency for the uptake of As(III) and As(V) ions over Fe3O4/CS/PEI nanocomposite at optimum conditions was found to be 99.5 and 99.7%, respectively. The experimental results were fitted using Freundlich’s and Langmuir’s isotherms. The data were more fitted to Langmuir isotherm providing a suggestion of monolayer adsorption with maximum adsorption capacity equal to 77.61 and 86.50 mg/g for the removal of As(III) and As(V), respectively. Moreover, linear regression coefficient (R2) indicated that the adsorption of arsenic ions over the synthesized magnetic nanocomposite obeyed pseudo 2nd order suggesting the chemisorption process. The reusability of the nanosorbent for arsenic uptake using sodium hydroxide as eluent was also assessed up to five cycles. Interestingly, Fe3O4/CS/PEI nanocomposite can be considered as a promising adsorbent for As ions’ removal from water and should be tested for the removal of other pollutants.