Removal of Fluoride and Arsenic by a Hybrid Constructed Wetland System

Delineating the fundamental attributes and traits of nature-based solutions in wastewater management

Water stress, exacerbated by population growth and climate change, necessitates sustainable wastewater management solutions that promote resource recovery and environmental protection. Nature-Based Solutions (NBS) offer a viable alternative to conventional wastewater treatment by leveraging natural processes for water purification and ecosystem restoration. However, the lack of standardised criteria for defining and evaluating NBS in wastewater management has led to inconsistencies in research and practice. This study conducts a systematic review of NBS applications in wastewater treatment, using Scopus and Web of Science databases, to delineate their fundamental attributes and establish a structured evaluation framework. By assessing NBS against seven key characteristics, i.e., natural processes, sustainability and resilience, biodiversity enhancement, multifunctionality, community and stakeholder involvement, cost-effectiveness and engineering approach, this study provides a comprehensive framework for distinguishing genuine NBS from other nature-inspired interventions. The findings contribute to improving the scientific rigour of NBS classifications, ensuring their scalability and fostering their integration into environmental management. This study offers a novel methodological approach to evaluating the effectiveness and applicability of NBS in wastewater management, facilitating their broader adoption and guiding future policy and research directions.

Enhanced fluoride removal in a novel magnesium-carbon micro-electrolysis constructed wetland through accelerated electron transport and anodic sacrifice

High fluoride (F-) content in the aquatic environment is a significant‌ issue affecting public health. Constructed wetlands (CWs) are believed to have unique potential for alleviating F- pollution in the aquatic environment. In this study, magnesium (Mg) and iron/nitrogen co-doped biochar (FeNBC) was used as the filler of a novel micro-electrolysis constructed wetland (WNME). Compared with the control, WNME significantly enhanced the F- removal efficiency from 19.1 ± 7.3 % to 54.1 ± 8.3 %, in which 90.7 % of total F- removal in WNME was attributed to the micro-electrolysis filler. The enhancement was attributed to the co-doping of iron and nitrogen, which improved the surface morphology, element distribution, and electrochemical performance of FeNBC. This led to an increase in the potential of FeNBC by 9.8 %, thereby increasing the potential difference within the Mg-C micro-electrolysis system. The Mg2+ release from anodic sacrifice in WNME was promoted, which led to an increase in MgF2 as the precipitate. Micro-electrolysis promoted the enrichment of electrochemically active bacteria in WNME, resulting in enhanced electron transfer and high F- removal efficiency. This study provided new insights for F- removal in CWs and would shed light on the optimization of micro-electrolysis CWs for F- removal from the aqueous phase.

Fate of arsenic in contaminated coastal soil induced by rising temperature and seawater intrusion

Temperature rising and seawater intrusion are expected to influence the hydrologic regime and redox conditions in coastal soil, and the fate and mechanisms of biogeochemical cycling of Arsenic (As) in the specific environment are poorly understood. This work was carried out in an anaerobic operating chamber by adding sulfate to simulate seawater intrusion under various temperature. Results demonstrated the microbial community diversity was influenced by temperature and the highest Shannon and lowest Simpson index were found at 28 °C. Firmicutes was the dominant bacteria, accounting for 81.16%-93.99%. Desulfosporosinus, with the proportion increasing with temperature, showed a significantly positive correlation with S2- for sulfate addition treatments. Actually, transformation of As was meditated by the concentration and valence of sulfur and iron in soil. The dissimilatory reduction of arsenic-bearing Fe oxides occurring in the initial stage, is suspected to be the primary driver of As release. Then, concentration of As declined in aqueous phase due to the reduction of sulfate, and the proportion of residual speciation of As in solid phase increased with temperature, ranging from 6.78% to 27.70%. The results displayed the reducing condition due to seawater intrusion and temperature change could regulate the release and sequestration of As in the coastal soil.

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.

Fluoride geochemistry in groundwater at regulated industrial sites

Few studies apply geochemical concepts governing fluoride fate and transport in natural waters to geochemical conditions at contaminated industrial sites. This has negative implications for designing sampling and compliance monitoring programs and informing remediation decision-making. We compiled geochemical data for 566 groundwater samples from industrial waste streams associated with elevated fluoride and that span a range of geochemical conditions, including alkaline spent potliner, near-neutral pH coal combustion, and acidic gypsum stack impoundments. Like natural systems, elevated fluoride (hundreds to thousands of ppm) exists at the pH extremes and is generally tens of ppm at near-neutral pH conditions. Geochemical models identify pH-dependent fluoride complexation at low pH and carbonate stability at high pH as dominant processes controlling fluoride mobility. Limitations in available thermochemical, kinetic rate, and adsorption/desorption data and lack of complete analyses present uncertainties in quantitative models used to assess fluoride mobility at industrial sites. PRACTITIONER POINTS: Geochemical fundamentals of fluoride fate and transport in groundwater are communicated for environmental practitioners. Fluoride is a reactive constituent in groundwater, and factors that govern attenuation are identified. Geochemical models are useful for identifying fluoride attenuation processes, but quantitative use is limited by thermodynamic data uncertainties.

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.

Bio-priming with a Novel Plant Growth-Promoting Acinetobacter indicus Strain Alleviates Arsenic-Fluoride Co-toxicity in Rice by Modulating the Physiome and Micronutrient Homeostasis

Sustainable remediation of arsenic-fluoride from rice fields through efficient bio-extraction is the need of the hour, since these toxicants severely challenge safe cultivation of rice and food biosafety. In the present study, we screened an arsenic-fluoride tolerant strain AB-ARC of Acinetobacter indicus from the soil of a severely polluted region of West Bengal, India, which was capable of efficiently removing extremely high doses of arsenate and fluoride from the media. The strain also behaved as a plant growth-promoting rhizobacterium, since it could produce indole-3-acetic acid and solubilize phosphate, zinc, and starch. Due to these properties of the identified strain, it was used for bio-priming the seeds of the arsenic-fluoride susceptible rice cultivar, Khitish for testing the efficacy of the AB-ARC strain to promote combined arsenic-fluoride tolerance in the rice genotype. Bio-priming with AB-ARC led to accelerated uptake of crucial elements like iron, copper, and nickel which behave as co-factors of physiological and antioxidative enzymes. Thus, the activation of superoxide dismutase, catalase, guaiacol peroxidase, glutathione peroxidase, and glutathione-S-transferase enabled detoxification of reactive oxygen species (ROS) and reduction of the oxidative injuries like malondialdehyde and methylglyoxal generation. Overall, due to ameliorated molecular damages and low uptake of the toxic xenobiotics, the plants were able to maintain improved growth vigor and photosynthesis, as evident from the elevated levels of Hill activity and chlorophyll content. Hence, bio-priming with the A. indicus AB-ARC strain may be advocated for sustainable rice cultivation in arsenic-fluoride co-polluted fields.

As and F − cooccurrence in drinking water: critical review of the international scenario, physicochemical behavior, removal technologies, health effects, and future trends

Drinking water contaminated with As andF − is increasingly prevalent worldwide. Their coexistence can have negative effects due to antagonistic or synergistic mechanisms, ranging from cosmetic problems, such as skin lesions and teeth staining, to more severe abnormalities, such as cancer and neurotoxicity. Available technologies for concurrent removal include electrocoagulation ~ adsorption > membranes > chemical coagulation > , and among others, all of which have limitations despite their advantages. Nevertheless, the existence of competing ions such as silicon > phosphate > calcium ~ magnesium > sulfate > and nitrate affects the elimination efficiency. Mexico is one of the countries that is affected by As andF − contamination. Because only 10 of the 32 states have adequate removal technologies, more than 65% of the country is impacted by co-presence problems. Numerous reviews have been published concerning the elimination of As orF − . However, only a few studies have focused on the simultaneous removal. This critical review analyzes the new sources of contamination, simultaneous physicochemical behaviors, available technologies for the elimination of both species, and future trends. This highlights the need to implement technologies that work with actual contaminated water instead of aqueous solutions (55% of the works reviewed correspond to aqueous solutions). Similarly, it is necessary to migrate to the creation of pilot, pre-pilot, or prototype scale projects, because 77% of the existing studies correspond to lab-scale research.

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

A laboratory-scale horizontal subsurface flow constructed wetland system was used to quantify the arsenic removal capacity in the treatment of highly acidic, arsenic and metal-rich water: pH ≈ 2, Fe ≈ 57 mg/L, Pb ≈ 0.9 mg/L, Zn ≈ 12 mg/L. The system was operated in two stages, being As ≈ 2.1 mg/L in stage one, and ≈ 3.7 mg/L in stage 2. Limestone and zeolite were employed as main supporting media to build non-vegetated and vegetated cells with Phragmites australis. The system was very effective in the removal of arsenic and iron (> 96%), and lead (> 94%) throughout the whole experimental period, having the four treatment types a similar performance. The main effect of the media type was on the pH adjustment capacity: limestone cells were able to raise the pH to ≈ 7.1, whereas zeolite cells raised it to ≈ 3.8. The contribution of plant uptake to the overall removal of As, Fe and Zn was minor; accounting for less than 0.02%, 0.07% and 0.7% respectively. As such, pollutants were mainly retained in the wetland beds. Our results suggest that limestone is recommended over zeolite as wetland medium mainly due to its neutralization capacity.