Ultra-high arsenic adsorption by graphene oxide iron nanohybrid: Removal mechanisms and potential applications
Graphical abstract
Introduction
Arsenic (As) is predominately a naturally occurring (geogenic) metalloid present in water (Meharg and Zhao, 2012). Inorganic arsenic in both As(III) and As(V) are the most prevalent species in groundwater. The maximum contaminant level (MCL) for total arsenic in drinking water is 10 μg/L (USEPA, 2001; WHO, 2003). Arsenic contamination of drinking water is a major public health concern across the globe and has affected more than 140 million people across 50 countries with Bangladesh, India, Argentina, Canada, Chile, Japan, and Taiwan being most affected (Murcott, 2012; WHO, 2018). About 2.1 million people in the United States who rely on domestic wells for their drinking water are in danger of facing arsenic contamination (>10 μg/L) (Ayotte et al., 2017). Excess arsenic in drinking water cause several health problems including skin lesions, respiratory problems, neurological complications, and circulatory disorders (Chen et al., 2009). Consumption of water high in arsenic may lead to cancers of skin and internal organs (liver, kidney, lung, and bladder) (WHO, 2018).
While adsorption is the most adopted method for arsenic removal, coagulation, flocculation, precipitation, ion exchange, and membrane filtration are also used. An ideal adsorbent should have high adsorption capacity, affinity for both the inorganic arsenic species (As(III) and As(V)) and should be effective under relevant environmental conditions.
Among several adsorbents, iron (Fe) based adsorbents are very effective and widely used to remove arsenic (Hao et al., 2018). Nanomaterials, mostly nano magnetite (M) and nanoscale zero-valent iron (NZVI) (Bezbaruah et al., 2013; Ling and Zhang, 2014; Tucek et al., 2017; Xu et al., 2019), are effective for arsenic removal because of their very high specific surface area and good adsorption capacity. However, these particles agglomerate easily and get oxidized rapidly (NZVI) (Krajangpan et al., 2012; Stefaniuk et al., 2016). Embedding iron nanoparticles (FeNPs) on sheets of carbonaceous materials enhances aqueous dispersion of the nanoparticles (Ma et al., 2013; Mortazavian et al., 2018), and graphene-based materials are found to be one of the most promising carbonaceous materials for such applications (Wang et al., 2013; Yoon et al., 2016). Graphene oxide (GO) based nanohybrid produced with iron nanoparticles deposited on GO showed improved dispersion behavior in water (Chandra et al., 2010; Huong et al., 2016; Yoon et al., 2016). GO is a 2D carbon sheet with sp2 hybridization with a very high specific surface area (320–940 m2 g−1) (Gao, 2015; Perreault et al., 2015). GO also contains a large number of hydrophilic groups —OH, — COOH, CO), and so has good dispersibility in aqueous media (Gao, 2015; Perreault et al., 2015). The functional groups in GO sheet also act as the nucleation sites for nanomaterial formation and facilitate a higher number of nanoparticles to be dispersedly deposited on the GO surface (Wang et al., 2010; Tang et al., 2011). Dispersed deposition of nanoparticles ensures that the surface area of each deposited nanoparticle is available for reaction with the target contaminants. Such GO-nanohybrids are reported to be good adsorbents for various contaminants (Wang et al., 2013; Guo et al., 2014).
The use of GO-iron nanohybrids are reported for metal and metalloid removal including arsenic (Luo et al., 2012; Guo et al., 2014; Hoan et al., 2016; Ren et al., 2018). The most reported GO-iron nanohybrid for arsenic removal is GO-Fe3O4 (Chandra et al., 2010; Yoon et al., 2016; Liang et al., 2019). There is also limited reporting on the use of GO-Fe0 nanohybrid for arsenic removal (Wang et al., 2014). The reported GO-iron nanohybrids (SI, Table A1) have shown limited arsenic removal capacity (6–180 mg/g) and that limits the potential life span of the arsenic removal systems to be fabricated with these nanohybrids. To enhance the removal capacity, an iron-based nanoparticle decorated on GO surface can potentially be used. GO-Fe nanohybrids offer such an architecture where the nanoparticles are well dispersed (less agglomerated) and, hence, will have enhanced contaminant removal efficiency. The GO layer will mediate electron transfer through initial storage of released electron (due to iron oxidation) and late release of electrons back to the iron nanoparticles. If a core-shell structured iron nanoparticle is used with GO, then the core-shell structure will be protected due to active electron transfer and effective life of the GO-Fe nanohybrid will be extended. For the ease of operation and maintenance, we need a treatment system that can run for a longer period of time before any maintenance intervention is needed. Further, the mechanisms of arsenic removal by these hybrid materials are not well investigated and understood.
In this study, we synthesized a GO iron nanohybrid (GFeN) using a sol-gel process where iron/iron oxide (Fe/FexOy) nanoparticles were decorated on the surface of GO. The new material was tested for its arsenic removal efficiency at environmentally relevant conditions and its field application potential was evaluated. Based on reaction kinetics, isotherm parameters, and characterization information, we have elucidated on the possible arsenic removal mechanisms. We also investigated the potential role of the GO sheet in arsenic removal by GFeN.
Section snippets
Materials and supplies
Graphene oxide in water (4 g/L, monolayer content >95%) was obtained from Graphena, (Spain), ferrous sulfate (FeSO4·7H2O, >99.5% pure), sodium borohydride (NaBH4, >97% pure) and other chemicals were reagent grade and purchased from VWR (USA). All chemicals were used as received unless otherwise specified. As(III) and As(V) solutions used in this experiment were prepared using individual 1000 mg/L standard stock solutions (Environmental Express, USA). Deoxygenated deionized (DDI) water was used
Material characterization
TEM micrographs show that the fresh (unused) GO sheets (Fig. 1a) are irregular in shape and a few micrometers in size (∼0.51 μm in the shorter direction and ∼4.10 μm in the longer direction) and folded in nature. Similar observations were made by others (Wang et al., 2014; Yoon et al., 2016). The bare FeNPs are found to be spherical in shape and clustered together (diameter = 15.3–65.3 nm, n = 31, Fig. 1b). GFeN (Fig. 1c) has nanoparticles decorated on the GO sheets and the particles are well
Practical significance
The relative high arsenic removal capacity of GFeN (>300 mg As/g for both the species) and rapid reaction kinetics are very significant for possible field applications of the nanohybrid. Assuming that a typical four-member family needs a minimum of 20 L of drinking water per day, the amount of GFeN needed to treat arsenic contaminated water to meet daily water demand will be 10 g per year (detailed calculations in Section A18, Table A6). It is important to note that this number is calculated
Conclusions
In this paper, we have reported an easy to adopt synthesis process for graphene oxide-iron nanohybrid (GFeN). GFeN exhibits very high adsorption capacities for As(V) (431 mg/g) and As(III) (306 mg/g) compared to other available nanohybrid sorbents (reported adsorption capacities of 12–240 mg/g, Table 2). At environmentally relevant arsenic concentrations (up to 140 μg/L), GFeN could bring down the effluent arsenic concentrations to below the MCL (10 μg/L) within 10 min. The adsorbent works for
CRediT authorship contribution statement
Tonoy K. Das: Conceptualization, Methodology, Data curation, Formal analysis, Visualization, Writing - original draft, Investigation. Tamil S. Sakthivel: Visualization, Formal analysis, Writing - original draft. Aadithya Jeyaranjan: Data curation, Formal analysis. Sudipta Seal: Supervision, Resources, Writing - review & editing, Funding acquisition. Achintya N. Bezbaruah: Supervision, Resources, Validation, Writing - review & editing, Project administration, Funding acquisition.
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
This work was funded by National Science Foundation (NSF Grant# CBET- 1707093, PI: Bezbaruah), USGS-North Dakota Water Resources Research Institute, and North Dakota State University (Grand Challenges Initiative). Tonoy Das received funding (fellowships) through NS-ICAR-IF from Indian Council of Agriculture Research and North Dakota Water Resources Research Institute (NDWRRI). Electron microscopy was done at the NDSU Core Laboratory (NSF Grant# CMMI-0821655). Sudipta Seal acknowledges the NSF
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