Nanostructured electrochemical sensor applied to the electrocoagulation of arsenite in WWTP effluent
Introduction
The adverse effects that pollutants released in the environment have on humans, animals, and ecosystems has promoted a change in the attitude towards this issue worldwide. Different species, such as emerging organic pollutants and persistent or heavy metals, have presented mutagenic, toxic, and carcinogenic effects that can endanger human health and water (Zhang et al., 2020). Arsenic (As) is one of the most toxic substances with a high environmental prevalence. It is estimated that more than one hundred million people around the world are exposed to unsafe levels of As or its compounds (Rodriuez-Lado et al., 2013Rodriǵuez-Lado et al., 2013). Arsenic can be found in the environment as: i) Organic As species, including arsenobetaine, monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA), which have low toxicity but tend to bioaccumulate in plants, cereals, algae, and animals (Feldmann et al., 2009; Promchan et al., 2016) ii) Inorganic As species, predominantly pentavalent arsenic (arsenate, As5+), and trivalent arsenic (arsenite, As3+). Arsenite is one hundred times more toxic to human beings than arsenate (Jain and Ali, 2000) and can also bioaccumulate as organic species (Abid et al., 2019; Antonova and Zakharova, 2016; Raab et al., 2016). Many international agencies, such as the World Health Organization (WHO), the United States Environmental Protection Agency (US-EPA), and the European Environment Agency (EEA), have listed arsenic as a toxic substance of priority concern (EEA, 2013; Gorchev and Ozolins, 1984; US-EPA, 2001). Therefore, a permissible limit of 10 μg L−1 of inorganic arsenic in drinking water is highly recommended.
The anthropogenic contribution of arsenic to soil and groundwater bodies is caused by mining, dye manufacturing, and agriculture (Garcia-Costa et al., 2021; Montefalcon et al., 2020), while naturally occurring arsenic contamination in water bodies derives from the dissolution of As-minerals such as arsenopyrite, enargite, realgar, and tennantite (Guo et al., 2010; Xie et al., 2013).
The high toxicity of inorganic arsenic makes it necessary to ensure access to safe water. Considering the problem above mentioned, a variety of electrochemical treatments has been studied in different types of wastewater and environmental matrices, in which oxidation methods, such as anodic oxidation (Espinoza et al., 2018), electro-Fenton (Thiam et al., 2018), and photo electro-Fenton (Vidal et al., 2018) stand out. Separation methods such as electrosorption (Ma et al., 2016; Penke et al., 2020) and electrocoagulation (Sık et al., 2015), are also employed with this purpose. In the treatment of water contaminated by arsenic, different methodologies such as adsorption (Asere et al., 2019), membrane filtration (McBeath et al., 2021), chemical coagulation (Ungureanu et al., 2015), and electrocoagulation (EC) (Syam Babu and Nidheesh, 2021) have been studied.
In recent years, EC has proved to be an efficient process to treat groundwater and wastewater (Goren and Kobya, 2021; Nidheesh and Singh, 2017). Several advantages, such as low sludge production, easy system set-up and operation, as well as inexpensive operating costs, have increased the interest in EC. In addition, EC can be applied directly in contaminated water if its conductivity and pH values are adequate (López-Guzmán et al., 2019). EC uses two metal electrodes that are connected to an external power source immersed in polluted water. An electric current is applied to the system, which promotes the oxidation process at the anode, generating metal cations (Mn). Simultaneously, H2O molecules are reduced to hydroxide ions (OH−) at the cathode. Pollutants are neutralized by coagulating agents [M(OH)n type], which collide, promoting the formation and growth of flocs that induce the removal of pollutants such as arsenic species, mainly through the absorption and precipitation of insoluble arsenic compounds (Valentín-Reyes et al., 2022). Several studies have dealt with the depletion of arsenic in groundwater/wastewater using aluminum or steel as sacrificial electrodes (Goren et al., 2020; Sandoval et al., 2021).
In turn, a variety of analytical methods has been developed to detect and quantify organic and inorganic arsenic species at low concentrations (Petursdottir et al., 2015; Urgast et al., 2014). For instance, inductively coupled plasma (ICP) allows for performing multi-element analyses, achieving trace and ultra-trace detection limits (Jr and Township, 2016). ICP has been compared to flame atomic absorption spectrometry coupled with hydride generation or with a graphite furnace as a standard technique for the control of environmental As (Hung et al., 2004; Jain and Ali, 2000). However, ICP-based methods exhibit the disadvantages (Ma et al., 2014) of not being useful for the study of numerous samples or the performance of real-time and on-site analyses. In recent years, the use of electrochemical sensors not only has allowed for conducting in-situ, but also limits of detection (LD) analyses with an order of magnitude as high as μg mL−1 and ng mL−1 (Bullen et al., 2020; Trachioti et al., 2019).
In this work, a hybrid structure of aminated multiwall carbon nanotubes (MWCNT) and gold nanoparticles (AuNP) was characterized to be applied as a working electrode for arsenite analysis at parts per billion (ppb). To this end, a new electroanalytical methodology was developed to monitor the removal of arsenic by means of the EC treatment in an actual secondary effluent from a treatment plant in order to ensure that its levels are safe for human consumption and industrial use.
Section snippets
Reagents
All chemicals used in this study were of analytical reagent grade. Arsenite trioxide, potassium chloride, potassium ferrocyanide, potassium ferricyanide, tetrachloroauric acid solution, sodium citrate dihydrate, sodium hydroxide, and boric acid were purchased from Sigma-Aldrich® (Santiago de Chile, Chile). Glacial acetic and orthophosphoric acids of EMSURE® grade were provided by Merck® (Santiago de Chile, Chile). Pure and functionalized multiwalled carbon nanotubes (MWCNT) and alumina
Electrochemical studies of MWCNT/AuNP-modified electrodes
Cyclic voltammetry studies were carried out using MWCNT, MWCNT-COOH, and MWCNT-NH2 modified glassy carbon electrodes to define the adequate electrochemical performance for the determination of arsenite. It should be noted that an electrochemical preconcentration stage was previously conducted by means of the application of a −500 mV potential for 100 s. A pH analysis was performed between 2.0 and 7.0 to find the highest current response. This study was performed using a 0.1 M Britton-Robinson
Conclusions
The new MWCNT-NH2/AuNP sensor developed in this work showed an improvement in terms of signal-to-noise ratio, sensitivity, and selectivity for the electrochemical signal of As3+, compared to other reported sensors. The application of an electrochemical methodology for arsenite quantification in a complex environmental matrix showed high sensitivity (19.58 nA L μg−1) and sufficient LD (0.39 μg L−1), confirming the potential of electroanalytical methodologies to monitor/control arsenite
Author contributions
Samuel Piña designed the experiments, discussed the results and made a first draft of the paper. Miguel A. Sandoval designed the electrocoagulation experiments, discussed the results and made a first draft of the paper. Paola Jara-Ulloa designed the electroanalytical experiments. David Contreras data treatment. Natalia Hassan performed the experiments and the data treatment. Oscar Coreño data treatment. Ricardo Salazar conceived, designed the experiments, discussed the results and made a
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We are grateful to DICYT-USACH 392102SG-AC, ANID/FONDAP/15110019 SERC Chile and ANID PhD fellowship N° 21190322 awarded to Samuel Piña Hodges. Miguel A. Sandoval is grateful to Agencia Nacional de Investigación y Desarrollo (ANID-FONDECYT, Chile) for granting the postdoctoral scholarship N° 3200274. We thank Juan Ernesto Ornelas Amaro from Centro de Innovación Aplicada en Tecnologías Competitivas (CIATEC) for his help in the FTIR analysis.
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