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Heavy metals in water: nano-remediation and quantification


To cite this article  use:  Reyes, Y., Torres, O. Heavy metals in water: nano-remediation and quantification.  J. Nano Sc. Tech, 4(2016)47-52

Water contamination has become a serious problem, since water contaminants -like heavy metals- have increased their concentration during the last century

Yulieth C. Reyes

Omar E. Torres

Water quality is an important aspect when developing different domestic, economic or industrial activities. Nevertheless, water contamination has become a serious problem, since water contaminants  -like heavy metals- have increased their concentration during the last century. Recently, heavy metals, such as mercury, arsenic, cadmium and lead, among others, have been found in natural sources of drinking water like groundwater, lakes, reservoirs, rivers and canals.

Water is the most important natural resource because it provides sustenance to plants, animals and humans.  On Earth, water is the most common substance, covering over 70% of the planet. In the surface, ocean and sea water represents the 96.54%; the 2.5% remaining is distributed in ice caps and glaciers (1.74%), groundwater (1.69%), permafrost (0.022%), lakes (0.013), soil (0.001), atmosphere (0.0009%), marsh (0.0008%), rivers (0.00015%) and biota (0.00008%).

Water quality is an important aspect when developing different domestic, economic or industrial activities. Nevertheless, water contamination has become a serious problem, since water contaminants, like heavy metals, have increased their concentration during the last century.  Recently, heavy metals, such as mercury, arsenic, cadmium and lead, among others, have been found in natural sources of drinking water like groundwater, lakes, reservoirs, rivers and canals. These metals also represent a serious problem for food security issues. Besides, these pollutants are absorbed by leaves and roots of plants as well as by animal tissues [1-2].

Despite occurring naturally, heavy metals pollution in water is mainly the result of inappropriate land use in farming practices (fertilizers, pesticides, sewage effluents and biosolids), industrial activities (mining, coal and petroleum combustion, indoor and urban environments, solid waste disposal), pollution of water reserves, and anthropogenic activities [3-4].  Bioavailability in soil-plant systems, and bioaccumulation in aquatic systems are problems resulting from that misuse [5].

Heavy metals are non-biodegradable and their ecotoxicological effects seriously affect human health. Among the problems caused by heavy metals, such arsenic, mercury, cadmium and lead, we have cancer (liver, lung and blood, especially), neurological and visual damages, negative affections in the immune system, allergies, cardiovascular toxicity, respiratory problems, dermatitis, diabetes and anemia [6].

Different specialized agencies in health and environment matters have established maximum allowable limits of heavy metals in drinking water, for example the World Health Organization (WHO) recommends a maximum concentration of 0.001mg/L for mercury. However, several academic reports have given an account of concentration figures above the maximum recommended for those metals, and have suggested the use of different strategies and new technologies, which embrace several engineering disciplines and applied sciences, for remediation, measurement and quantification of heavy metals in polluted water [7].

Some traditional water treatment tech-niques, such as physical remediation (soil washing, encapsulation and vitrification), chemical remediation (precipitation, flocculation, ion exchange, membrane filtering and solidification), electrokinetic techniques and barrier systems, have been used for remediation of residual water. These techniques allow the removal of macroscopic particle mainly, but have serious problems with microscopic ones [2]. Moreover, these technologies have some other problems like low sensibility, cost, response time and portability [8-9].

Nanoscience and nanotechnology have had a great impact when dealing with drinking water, as detection and quantification of heavy metals and remediation of residual water processes have significantly improved [10].  Because of their very specific characteristics (shape, size and physical properties), nanomaterials have an important role in the physico-chemical behavior of small-scale materials [8].   

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Nanotechnologies for remediation an quantification of heavy metals

In Europe and North America, the use of nanoscience and nanotechnology for quality water issues has been in the spotlight of the scientific community [11]. Nanotechnology can help to remove organic and inorganic microscopic pollutants in residual water. The adsorption and the use of membranes with nanomaterials are two techniques employed for removal of heavy metals, and surface plasmon resonance (SPR) sensors in detection and quantifications of metal ions.

Adsorbents

Adsorption is commonly used for remediation of organic and inorganic pollutants in residual water. Different types of nanomaterials, such as nanosorbents, zeolites and dendrimers, have been used for removal of heavy metals in residual water with exceptional absorption properties [2].

For heavy metals removal from aqueous systems, nanosized ferric oxides, magnesium oxides, aluminum oxides, titanium oxides  and cerium oxides are commonly used as adsorbents [4].  These nanomaterials have also been employed to remove heavy metals like arsenic, lead, mercury, copper, cadmium, chromium and niquel [12].

Efficiency of conventional adsorbents is usually limited by the surface area on active sites, the lack of selectivity and the adsorption kinetics [2]. The growing development of nanoadsorbents have greatly benefited from their high specific surface area, which is associated with their short intraparticle diffusion distance, tunable pore size, surface chemistry and the characteristics of the sorption site [12]. CNTs,  zeolites and dendrimers are common nanoadsorbents used in residual water to absorb heavy metals. Natural zeolites are aluminosilicates with skeletal structure, containing free spaces filled with large ions and water molecules. These compounds have unique properties such as the presence of highly efficient adsorbents of various compounds (gaseous mixtures and dissolutions), cation exchangeable property, catalytic property; besides,  each type of zeolite adsorbs molecules of a specific size [13]. Recently, different researches on nanosorbents have been carried out, including areas like nanobeats, nanocomposites, magnetic nano adsorbents and nanofibrous matrices [14].    

Activated carbon is an adsorbent material, commonly used for removal of heavy metals from aqueous solutions [15].  Activated carbon is a low cost material with a basic form of graphite; nonetheless, its microporous morphology causes limitations when adsorbing, for example, the obstruction of the micropores [9]. In spite of that, many researchers have reported the removal of  ions  of  Co(II), Cd(II), Ni(II), Pb(II), Cr(III), Cr(V) and Cu(II) with activated carbon, synthesized from peat, coconut shell and coal [2].  Lately, metal based nanomaterials proved to be better in removing heavy metals than carbon activated; for example, TiO2, nanoparticles and nanosized magnetite for arsenic adsorption [7].

Membranes

Membrane filtration is a common process in water treatment nowadays. However, the filtration efficiency depends on the material used for filtering16. The efficacy of nanofiber membranes in retention of heavy metals, such as Al and Cu(II),  in residual water has been highlighted by many researchers, but their applications are still unexploited [17]. Membranes have high specific surface area and porosity, and form nanofibers mats with complex pore structures [12]. Nano-Ag, TiO2, zeolites, magnetite and CNTs are some nanomaterials employed for membranes because of their potential properties like hydrophilicity, low toxicity  for humans, high mechanical and chemical stability, high permeability and selectivity and photocatalityc activity [17].

Sensors

Recently, nanosensors have been designed for sensing and monitoring tasks of highly toxic heavy metals, like arsenic and mercury. “These nanosensors are characterized by their (i) sensibility for detecting metals at low concentrations, (ii) low cost for monitoring and mapping in indefinite space and time, (iii) portability, that permits in situ measurement, and (iv) autonomy for measuring over an extended period of time.   Nanosensors are designed to take advantage of the capacity of modifying physico-chemical behavior in shape, size and composition terms that nanomaterials have “ [8].

Some sensors employed for detection, quantification and monitoring tasks of heavy metals at different concentrations are referred to as nanobiosensors. These devices analyze samples called analytes, consisting of a biological receptor for detecting  specific substances, a transducer for measuring the reaction of recognition  and an amplifier for sending the quantification signal [18].

Surface plasmon resonance (SPR) sensor based on Kretschmann configuration follows as operating  principle the reflectivity index of a metallic box in contact with the medium of the analyte [10]. The operation configuration of these sensors contains a glass substrate coupled to a prism with the same reflectivity, onto which a thin metallic layer, usually gold, is deposited. Then a beam laser light collides on the gold surface at a specific angle. Afterwards, the beam is collected by a photodetector: the measurement of the changes in the intensity of the reflecting beam, produced by the variations of the refraction index in the gold surface, allows to correlate them with the analyte concentration.

Gnano, research group  of  the Geophy-sics Institute, Pontificia Universidad Javeriana (Bogotá, Colombia) has, following the Kretschmann configuration, developed a sensor for sensing and monitoring in situ arsenic concentration in real time. This  system, which can operate autonomously, requires that water samples are free of chemical agents of particulate material, responsible for interference according to the measures [10,19].

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Figure 1. Operation scheme of a biosensor.

The protocol employed was design and implemented by using nanoestructures gold-surface, type Au(111), and molecular arrangements capable of immobilizing and quantifying materials such as arsenic [20].  The system also has a pre-treatment system for samples collected during the measurement. It consists of two compartments: the first removes particulate material and  chemical species, different from arsenic; the second only allows the output of arsenic ions [9].

Researchers use molecular dynamics in order to assess the nanobiosensors and biosensors behavior, aiming at analyzing the interactions between the analytes of interest and the self-assembled monolayer. These monolayers, over metallic sustrates, are an important mechanism for detection and quantification of heavy metals in drinking water. Nowadays, several research groups are working on controlled processes in laboratory to develop devices based on self-assembled monolayers.

Toxicology 

Nanoparticles in the environment are a topic of interest for researchers.  Nowack and Bucheli recommend to understand the behavior, toxicity and reactivity of nanoparticles [21]. Some nanoparticles employed for removal of pollutants, purification, remediation and removal of toxic elements and compounds  in water, such as nanoparticles of Fe3O4, Fe, Ni, Co, Fe, Pd, SiO2, TiO2, can affect chemically and biologically the environment when badly used [11].   

Studies have also shown that nanoparticles containing zinc and aluminum have toxic effects on germination and growth of roots in relevant plant species; furthermore, they can affect human health, for example, in 1977, British scientists showed that titanium dioxide and zinc oxide nanoparticles contribute to the formation of free radicals in skin cells, which can harm the DNA [11].

      

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Figure 2. Water contamination has become a serious problem, since water contaminants, like heavy metals, have increased their concentration during the last century.

Undoubtely, the study of nanomaterials is essential to keep on advancing in the development of techniques for polluted water remediation. At the same time, other fields have emerged to take advantage of nanomaterials technologies, for instance, nanotoxicology, which is relevant since removal remains after remediation processes could also be a source of pollution.

References

[1] Bradl, H.B. Interface science and technology. Volume 6. Heavy metal in the Environment. (2005).

[2] Baruah, S., Najam Khan, M. & Dutta, J. Environ. Chem. Lett. 14, 1–14 (2015).

[3] Chen, Y. et al. Ecotoxicol. Environ. Saf. 98, 324–330 (2013).

[4] Hua, M. et al. J. Hazard. Mater. 211-212, 317–331 (2012).

[5] D’Ambrosio, M. C. Evaluación y selección Tecnol. Dispon. para remoción arsénico 123–136 (2005).

[6] Nava-Ruíz, C. & Méndez-Armenta, M.  Arch. Neurociencias 16, 140–147 (2011).

[7] Amin, M. T., Alazba,  a a & Manzoor, U. A Review of Removal of Pollutants from Water / Wastewater Using Different Types of Nanomaterials. (2014).

[8] González, E. in El problema de contaminación por Mercurio. Nanotecnología, retos y posibilidades para Medición y Remediación. 167–199 (2015).

[9] Reyes, Coy, E. Yate, L. Jurga, S and E. González, Y. . ACS Sensors DOI: 10.1021/acsphotonics.5b00667 (2016).

[10] Salinas, S.  et al.  Sensors and Transducers. 183, 97–102 (2014) .

[11] Bystrzejewska-Piotrowska, G., Golimowski, J. & Urban, P. L. Waste Manag. 29, 2587–2595 (2009).

[12]Qu, X., Alvarez, P. J. J. & Li, Q. Water Res. 47, 3931–3946 (2013).

[13] Burmańczuk, A. et al. J. Elemntology 20, 803–811 (2012).

[14] Kampalanonwat, P. & Supaphol, P. ACS Appl. Mater. Interfaces 2, 3619–3627 (2010).

[15] Kobya, M., Demirbas, E., Senturk, E. & Ince, M. Bioresour. Technol. 96, 1518–1521 (2005).

[16] Sang, Y., Gu, Q., Sun, T., Li, F. & Liang, C. F J. Hazard. Mater. 153, 860–866 (2008).

[17] Albuquerque, U. P. et al. J. Ethnobiol. Ethnomed. doi:10.1186/1746-4269-9-72 (2013).

[18] Sáez, J. S. del R. Desarrollo de un Biosensor fotónico de alta sensibilidad basado en interferómetros Mach-zehnder integrados en tecnología de Silicio. 3–7 (2004).

[19] Salinas. N, Mosquera. L, Yates. E, Coy. G, Yamhure & E.González, S. Autonumus nanosensor system for monitoring and quantifying arsenic in water. Proc. 2014 NSTI Nanotechnol. Conf. Expo, NSTI-Nanotech 3, 170–173 (2014).

[20] Mosquera, N. J. Nano Cienc. y Tecnol. 2, 46–48 (2014).

[21] Nowack, B. & Bucheli, T. D.   Environ. Pollut. 150, 5–22 (2007).

__________________

Yulieth C  Reyes  MSc.

Grupo de Nanociencia y Nanotecnología, Instituto Geofísico, Facultad de Ingeniería, Pontificia Universidad Javeriana. Bogotá, Colombia. E-mail: yreyes@javeriana.edu.co

Omar E  Torres MSc.

Grupo de Nanociencia y Nanotecnología, Instituto Geofísico, Facultad de Ingeniería, Pontificia Universidad Javeriana. Bogotá, Colombia. E-mail: torresomar@javeriana.edu.co

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