Interaction of Heavy Metal Ions With Nanomaterials: A Remediation Process

Introduction

Today approximately 663 million people lack access to safe drinking water all around the world (WHO, 2015). Heavy metal contamination is the major contributor to the world drinking water crisis, affecting millions of people worldwide. Major contributors for heavy metal pollution are lead (Pb), cadmium (Cd), arsenic (As), mercury (Hg) and chromium (Cr) as are most toxic and nonbiodegradable. Their presence above permissible limits in body can cause severe damages to vital organs of the body, such as kidney, liver and brain, reproductive and nervous system (Moreno-Castillaet al., 2004; Zhao et al., 2011). These heavy metals are widely utilized by many industries such as metal finishing, iron and steel, metallurgical, mining, battery producing industries etc. and are released in various water sources (Chen et al., 2007). Various technologies reported for remediation of metal ions from wastewater are solvent extraction, reverse osmosis, separation by membrane, ion exchange etc (Walsh et al., 1994; Xing et al., 2007; Ersahin et al., 2012). The available techniques have disadvantages such as high cost, high required investment and disposal of metal sludge; therefore, efforts have been made to synthesis low cost materials for removing heavy metals from aqueous solutions. Nowadays, nanomaterials (NMs) are recommended as effective, low cost and eco-friendly substitute to available treatment materials, in both resource conservation and environmental remediation (Zare et al., 2013; Gupta et al., 2015). The elimination of heavy metals from polluted sites using nanomaterials formed by plant, fungi and bacteria with the assistance of nanotechnology is called nanobioremediation. Green route for synthesis of nanoparticles has developed markedly to form novel materials that are eco-friendly, cost effective and stable. Although nanoparticles can be manufactured through a range of conventional processes but the biological route of synthesizing is more beneficial because of ease of fast production, low toxicity, control of size characteristics, low cost and eco-friendly approach (Mei et al., 2014). In the development of various remediation technologies for heavy metals, adsorption has already been proven to be an effective approach towards contamination removal (Tripathy et al., 2006; Onyangoa et al., 2006). The adsorption process mainly depends upon the applicability of the absorbent. The quality of the effluent generated is also better than the rest of the processes because the adsorbent has a high affinity towards the metal ions. Various types of adsorbents such as zeolites, activated carbons, chelating materials and nanoparticles have been developed for the remediation applications. The main selection parameters of adsorbents are the high adsorption capacity, fast kinetics and low cost (Wang et al., 2010; Cai et al., 2013; Gupta et al., 2015). With the development of technology, nanomaterials are utilized for heavy metal remediation due to their more surface area and low-cost synthesis and proved efficient adsorbents. Among all reported nanomaterials, remediation of heavy metals ions by metal oxide based nanocomposites and iron oxide based magnetic nanomaterials has been broadly utilized. Nano sized iron oxide particles exhibit super paramagnetism, an additional special property from common nanoparticles. Super paramagnetic nanoparticle have large surface area, are biocompatible, less toxic, chemically inert, offer small diffusion resistance and their surface can be modified with organic molecules, inorganic ions or some functional groups, which render them surfaces having good potential for removing heavy metals (Hua et al.,2012). Nanotubes synthesized are used for the removal of Cu (II), Zn (II), Hg (II), Ni (II), Cr (VI) and Pb (II). Manganese oxides (MnO) nano-particles, zinc oxide (ZnO) nano-particles, magnesium oxide (MgO) nano-particles, carbon nanotubes (CNTs) and graphene based nano-adsorbents are utilized for the remediation of different heavy metal (Mubarak et al., 2014). Heavy metal removal from contaminated water through the use of nano-adsorbents is influenced by several parameters such as temperature, pH, adsorbent dose and contact period. The enhancement in contact period elevates the adsorption of heavy metals from polluted water as it gives more time for the adsorption. At first, the adsorption of heavy metals is more due to the high concentration of metals, which declines as time period increases due to blockage of active sites. Adsorption is also influenced by nanomaterials properties, such as surface charge, BET surface area, hydrophobicity and addition of new functional groups (Matlochová et al., 2013). There is no doubt of efficiency of utilization nano-materials in wastewater treatment; however, this technology has some serious downsides that need to be negotiated. Since nanoparticles might release into the environment during preparation and treatment processes, where they can accumulate for long time and cause serious risks. In order to reduce the health risk there is need a future research to prepare such catalysts having least toxicity to the environment. More work is required to re-evaluate the ecotoxicity potential for each new modification in catalyst and for existing materials. In addition, life cycle assessment of nanomaterials is crucially required to address their overall benefits and risks. Nanotechnology is rarely adapted to mass processes. Given that up until now, most of the nanomaterials have not been cost-competitive when compared with conventional materials such as activated carbon, and thus future applications will focus on efficient processes where only small quantities of nanomaterials are required. Moreover, further work is required on developing a cost effective methods of synthesizing nanomaterials and testing the efficiency at large scale for successful field application.