Heavy Metals Alleviation By Nano-Materials

Abstract

Heavy metals discharge from agricultural, domestic, industrial and municipal wastewater has become a serious threat for environment. During past couple of decades new class of adsorbents developed, has helped to overcome this threat. Due to their excellent potential in wastewater and industrial effluents treatment and distinct characteristics they have gained popularity. This review paper presents, nanoadsorbents used in removal of heavy metals, highlighting their efficiency and different mechanism tangled in the removal.

Introduction

Arsenic (Ar), cadmium (Cd), chromium (Cr), cobalt (Co), lead (Pb), mercury (Hg), nickel (Ni), and zinc (Zn) are heavy metals known for their persistency and toxicity. They are mostly non-biodegradable. Through manufacturing industries, mining operations, sludge disposal and refineries, they find their way to water bodies. They may be carcinogenic and mutagenic, so their presence in water may pretense serious threat to life and their presence above suggested limits can cause damage to vital organs of body, i.e. brain, kidney, liver, nervous and reproductive system (Goel, 2006). Since decades, the conventional methods of wastewater treatment employed such as evaporation, electrochemical treatment methods, lime coagulation, ion exchange, solvent extraction chemical precipitation and filtration method, reverse osmosis and redox reactions are reported as expensive and inadequate because of low metal removal efficiencies for high operational costs.

Adsorption is a common phenomenon in gaseous phase and is used well in wastewater treatment since ancient times. For decades, granular activated carbon because of its properties has been used for wastewater treatment. From wastewater the green sand, red mud, slags are industrial waste, found effective adsorbents for inorganic and organic materials including most heavy metals (Vigneswaran and Moon, 1999). Sea bacteria can produce dead or living cells for biosorption of trace heavy metals facilitated in detoxification of wastewater (Lopez-Cortes and Ochoa, 1999). Adsorbents of low costs prepared from agricultural waste showed very operative in elimination of heavy metals even at low concentrations from wastewater (Babel and Kurniawan, 2003). In wastewater treatment polymeric membranes used are cross linked poly sulfone, cellulose acetate materials and polyamide, and exhibit special characteristics i.e. high flux rate, chemical stability, better metal ion binding with chelating mechanism and resistance to ozone, ultraviolet rays and chlorine (Lakshmanan, 2013). Since few decades, new class of materials has been developed i.e. derivatives of carbon, ceramics, carbonaceous nonporous materials, micro-porous glasses, molecular sieves, polymers, silica nanoparticles etc. (Kumar, 2009). They have extended the interest of young researchers, due to cosmic scope of research in wastewater treatment using different materials with diverse characteristics of reuses and have high adsorption capacity for heavy metals from wastewater in comparison to materials used as adsorbents traditionally and commercially.

Nano Materials

With the introduction of scanning tunnel microscope and cluster science in 1980, Nano science and technology gained popularity (Mehta et al., 2015). Nano size materials range (1-100 nm) exhibit effective process parameters and diverse properties, because of large number of particles, specificity for pollutants, high surface to volume ratio, surface interactions and magnetic separation (Lakshmanan, 2013). Different nano-materials synthesized in recent times are nano-cages, nano-crystals, nanoflakes, nano-wires, silica nanoparticles, nano-fibrils, hetrofullerenes and hydroxyfullerenes. They can be synthesized from metals, their metal hydroxides and metal oxides (cobalt hydroxide), biopolymers and polymers, derivatives of carbon and ceramics etc (Dabrowski, 2001; Jaroniec and Sayari, 2002; Knetch and Wright, 2004; Kumar, 2009; Oaki and Imai, 2007; Chwastowska et al., 2008; El Saliby et al., 2008; Donald, 2010; Fi and Li, 2010; Liu et al., 2012; Srivastava, 2013; Heimann et al., 2015) by co-precipitation, sol gel method, direct oxidation, micro-emulsion, microwave colloidal etc. (Chen and Mao, 2007).

They find their use as metals in the field of electronics and medicine. Due to high chemical potential and activity, the application of nanoparticles is gaining devotion in wastewater treatment due to high adsorption capacity for heavy metal ions. Constants between phases, the rate of reaction between the phases and equilibrium concentrations can be defined by the chemical potential and thermodynamic activity are important parameters (Kaufman, 2002).

Magnetic nanoparticles

Super paramagnitism exhibited by nano sized iron particles, super paramagnetic nanoparticle are biocompatible, chemically inert, less toxic, have large surface area, offer small diffusion resistance and their surface can modified with some functional groups, organic molecules or inorganic ions, due to which they have good potential for removing different heavy metals. Surface modifications of nanoparticles make them; (a) specific and selective for heavy metal ions uptake by providing specific functional groups (-NH2, –SH, -OH and –COOH etc. can provide active sites for metal ion exchange) and reaction sites, (b) stable by preventing them from oxidation. Chemical interactions (chemical binding, complex formation and modified ligand combination) and physical interactions (vander waal and electrostatic interactions) are responsible for adsorption of metal ions the surface of adsorbent (Wang et al., 2012; Patwardhan, 2012). For mercury (Hg), metal removal efficiencies were compared for modified magnetite magnetic nanoparticles and magnetite modified with 2-mercaptobenzthiazole. For initial mercury concentration of 50 ng/mL about 43.47% removal showed by unmodified nanoparticles, while within 4 minutes about 98.60% of mercury removed by modified nanoparticles. Magnetic nanoparticles have tendency to change their properties and to accumulate.

Manganese iron oxide magnetic nanoparticles synthesized with amorphous oxide shells of manganese and cobalt, exhibited a negative charge over wide range of pH. This result in excellent adsorption capacities of 345.5 mg/g for Cd, 481.2 mg/g for Pb and 386.2 mg/g for Cu, and prevented their accumulation in aqueous solution (Ma et al., 2013). Ferrite coated apatite magnetic nanoadsorbent can be synthesized by co-precipitation method for Eu(III) ions removal from aqueous solutions Moussa et al. (2013). Magnetic nanoparticles possessed a crystalline structure with specific surface area 85.11 m2/g, mean particle size of 63 nm and high thermal resistance up to 600 °C. With adsorbent dose of 5 mg, within 12 hours the equilibrium was attained. At pH of 2.5 maximum adsorbed capacity was 157.14 mg/g. For the removal of arsenic from aqueous solution, ferrous oxide nano composites of cellulose can be synthesized by pot chemical co-precipitation method Yu et al. (2013). Magnetic nanoparticles had surface area of 113 m2/g and its sensitive magnetic behavior making it easily separable by application of magnetic field. Adsorption capacity to remove As (V) and As (III) were 32.11 and 23.16 mg/g respectively and data fitted well with the isotherm model (Langmuir and Freundlich).

By different experimentations with magnetic nanoparticles Cr (VI) can be removed from water Li et al. (2013). It was prepared by doping nitrogen with porous carbon (RHC-mag-CN), having super paramagnetic characteristics and specific area of 1136 m2/g. At pH=3 maximum Cr(VI) removal was 16 mg/g and the process described by Langmuir isotherm model. Modified magnetic nanoparticles can be synthesized by covalent immobilization of thiosalicylhydrazide on magnetite nanoparticle surfaces for heavy metal ions removal from industrial effluents. Its maximum adsorption capacities obtained were 107.5, 188.7, 51.3, 27.7 and 76.9 mg/g for Cd(II), Pb(II), Zn(II), Co(II), Cu(II) respectively. Other advantages of modified nanoadsorbants were ease of separation after treatment, reusability and environment friendly composition making them perfect for heavy metals removal from wastewater.

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Metal oxide nanoparticles

Nanoparticles can be produced from oxides of different metals such as aluminum, copper, iron, magnesium, manganese, silica and zinc. The main aim is to get nanostructures exhibiting diverse chemical and physical properties, different from single or bulk structured particles and due to this, the band gap between oxide particles decreases with decrease in size of particles cause the change in chemical reactivity and conductivity (Fernandez-Garcia and Jose, 2007).

Metal oxide nanoparticles express higher extent of adsorption in comparison with normal sized oxide, due to formation of ternary ligands or metal-ligands precipitation (Stietiya and Wang, 2014). Oxides of Cu, Fe and Zn as nanoadsorbents revealed the dependence of heavy metal removal efficiency on pH of the solution. Heavy metal ions removal from solution increased with pH, because of precipitation and formation of metal complexes due to covalent or ionic bonding, electrostatic interactions and nature of the functional group. The increasing the negatively charged sites, deprotonation of nano sorbent surface favored by increased pH of solution, and also favored the attraction forces between negatively charged sites and positively charged ions on adsorbents. The competitive adsorption occurred between competing H+ ions and metal ions in solution on lowering the pH of the solution (Mahdavi et al., 2012). Magnetite (Fe3O4) nano adsorbents were used to study the removal of lead ions from aqueous solution, the maximum adsorption capacity of lead is 36 mg/g and within 25-30 minutes the equilibrium was attained, and equilibrium data fitted well into isotherm models of Langmuir and Freundlich. This process is endothermic as temperature increased the lead amount adsorbed while thermodynamic studies indicated that the process was spontaneous Nassar (2012).

Zeolite

Zeolites are alumino-silicate minerals of alkaline or alkali earth metal. It consists of three dimensional networks of silicon dioxide tetrahedral and aluminate linked by partnership of oxygen atoms. The voids make up from 20-50% of crystal volume of zeolites (Mumpton 1978, 1996; Gottardi 1978). The natural zeolite application for water treatment is an encouraging technique in environment cleaning processes (Wang & Peng, 2010). The use of natural zeolites has engrossed on removal of heavy metals and ammonium through in exchange capacity and ion selectivity, ion exchange properties of zeolite trap undesirable metals and prevent from entering ecosystems and to food chain (Colella 1996) (Mumpton, 1985). Different studies report that powdered zeolites i.e. clinoptilolite, reduce the transfer of heavy metals such as; lead, copper and zinc from soil to plants (Mumpton, 1985). Untreated and Pretreated clinoptilolite zeolite is effective in removal of cadmium, lead, nickel and zinc from wastewater (Turkman et al. 2004).

In the study of zeolite impact on purification of acid mine waters; the results indicated that heavy metals were successfully up taken by zeolite materials in water purification process. Specific dose of zeolite material were applied according to heavy metals concentration. The zeolite increase pH, which in result enhance the efficiency of the decontamination process by bearing metal bearing soil phases to precipitate [Moreno et al. (2001)]. Heavy metals can be removed from contaminated water by applying a mixture of bacteria and HCl-activated clinoptilolite. The solution of bacteria and HCl-activated clinoptilolite is effective as water decontaminates and is successful in removing 98% of copper (Cu), iron (Fe) and Cobalt Mamba et al. (2009). Natural zeolite is effective to eliminate heavy metals effectively from industrial wastewater Loizidou and Townsend (1987).

Silica nanoparticles

In water purification techniques, silica is used in coating of nanoparticles, as silica coating activates the surfaces of nanoparticles having different functional groups due to presence of silanol groups on silica layers. In low pH conditions it protects nanoparticles from leaching. It also facilitates the nanoparticles with group specific ligands and non-specific moieties. The acidity of silica nanoparticles increases with increase in particle size and resulting in 5-20% ionization of silanol groups at neutral pH (Wang et al., 2012; Patwardhan, 2012). Nanostructured graphite oxide, silica/graphite oxide composites can be used to remove Cd, Cr, Ni and Zn ions from aqueous solutions Sheet et al. (2014). Heavy metals in the order Ni > Zn > Pb > Cd > Cr, adsorbed by nanostructured graphite oxide, and showed good results for Ni ions removal as per Langmuir adsorption isotherm model. Freundlich isotherm model recommended adsorption of monolayer type for heavy metal ions. Over other two adsorbents, graphite/silica oxide composite (3:2) was suggested for efficient adsorbent of wastewater treatment. Removal of Cd, Cr, Ni, Pb and Zn from waste water was studied with silica/activated carbon (2:3) nano-composite (average particle size 12nm), silica nanoparticles (appeared as white aggregate at 10 μm magnification) and activated carbon micro-particles (ACμPs) (average particle size 25μm). Out of these, for removal of Ni ions silica/AC (2:3) nano-composite showed best efficiency as compared to silica nanoparticles and ACμPs. Nickel ion concentration (30 mg/L), percentage of removal by silica nanoparticles, Si/AC (2:3) NC and ACμP were 70.30, 92.11 and 99.40 respectively and at concentration of (200 mg/L), the removal efficiencies were 60.1, 84.11 and 87.62 respectively (Karnib et al., 2014).

Modified amino silica nanoparticles used to study Cd, Cu, Hg, Pb, and Zn adsorption from aqueous solutions, were obtained after treatment with industrial silica fumes at 80 °C by HNO3. To functionalize their surface silane soupling agents were used and modifications done using chloro-acetyl chloride and 1,8-Diaminoaphalene, The results showed that the functionalized nanoparticles showed excellent results for Cu, Hg and Pb ions (Kong et al., 2014). Meso-porous silica nanoparticles synthesized by inserting silica magnetic nanoparticles with cetyltrimethylammonium bromide, and then followed by modification with silane coupling agent 3-aminopropyltriethoxysilane to eliminate Cr ions from aqueous solutions Araghi et al. (2015). According to experimental data the adsorption varied with pH. The Langmuir isotherm model revealed rise in adsorption capacity with temperature. The nanoparticles indicated regeneration, fast adsorption, easy separation from solution on application of reutilization and external magnetic field. Mesoporous silica nanoparticles MCM-41 which is grafted from derivatives of poly-amide, used as the supporting matrix was produced for separation of mercury from aqueous solution. It was characterized by high surface area and large pores, within 3 minutes this material extracted and separated trace mercury ions from aqueous solution in pH range of 3-11 (He et al., 2015).

Carbon nanotubes

These are engineered materials having unique properties, such as optical activity, surface morphology, electrical conductivity and mechanical strength. They are good adsorbents because of their light mass, high porosity, large specific area, strong interactions with pollutants and hollow structure (Dresselhaus et al., 2001). In carbon nanotubes adsorption sites are interstitial channels, outside surface, internal sites and grooves. On external sites equilibrium reached faster as compared to internal sites. The adsorption mechanism is attributed to chemical interactions between metal ions and surface functional groups of carbon nanotubes. Dependency on synthesis and purification carbon nanotubes contain –C=O, –OH and –COOH groups. By oxidation with Ni, Pt or Pd as catalysts more functional groups can be added to carbon nanotubes or can be removed from carbon nanotubes by heating at 2200 oC. Carbon nanotubes show preference for different hydrophobic groups, such as hexane, benzene and cyclohexane than alcohol which is hydrophilic group. On the functionalization, due to change in wettability of carbon nanotubes this preference can be reversed. This is due to the hydrogen atoms from functional groups of carbon nanotubes can get replaced by metal ions and cause the fall in pH of solution as more H+ ions released Lu et al., 2006; Gotovac et al., 2007; Gadhave and Waghmare, 2014). Maghemite nanotubes were used for the removal of Cu, Pb and Zn which can be synthesized by microwave irradiation method. Nanoparticles had magnetic saturation of 68.7 emu/g and surface area was 321.6 m2/g and they showed maximum adsorption capacities of 84.95 mg/g, 71.42 mg/g and 111.11 mg/g for Zn, Pb and Cu respectively. This kinetic data showed pseudo second order equation (Roy and Bhattacharya, 2012).

Carbon nanotubes sheets synthesized by chemical vapor deposition of cyclohexanol and ferrocene at 750 °C in the presence of nitrogen and functionalized with concentrated HNO3 and chitosan. These sheets remove Cu ions from aqueous solutions. After functionalization the capacity of carbon nanotubes sheets improved from 23.32 mg/g to 57.34 mg/g for initial 800 mg/L concentration of Cu. Adsorption behavior was described by pseudo-second order equations and Freundlich and Langmuir models (Tofighy and Mohammadi, 2015). Multiwall carbon nanotube-magnetite nanocomposites magnetic nanoadsorbent synthesized and showed maximum adsorption capacity for Pb and Hg removal, which is 65.40 mg/g and 65.52 mg/g respectively and surface area obtained was 97.15 m2/g (Zhang et al., 2012). Sulfonated multi-walled carbon nanotubes (s-MWCNTs) prepared by treating multi-walled carbon nanotubes (p-MWCNTs) at high temperature with concentrated sulfuric acid (H2SO4) to remove Cu ions from aqueous solution. Sulfonated multi-walled carbon nanotubes (s-MWCNTs) revealed adsorption capacity of 58.90% for Cu on sulfonation. This process was well explained by D-R and Freundlich models Ge et al. (2014). Magnetic multiwalled carbon nanotubes nanocomposite (MMWCNTs-C) were used for Ni ions removal from aqueous solution, its capacity was calculated as 2.1 mg/g. Nickel adsorption process on magnetic multiwalled carbon nanotubes nanocomposite was spontaneous and favorable thermodynamically. Equilibrium data indicated that, adsorption process was explained well by Langmuir isotherm model and pseudo-second-order kinetic model than Freundlich isotherm model (Konicki et al., 2015).

Conclusions

For sustainable growth of society in present scenario the biggest challenge is, the treatment of wastewater loaded with hazardous and toxic waste to make it reusable. To address this issue adsorption with nanoparticle/nano-materials has emerged as one of the potential technology. Its unique chemical and physical properties due to high density of surface and due to limited size make them proficient to revolutionize the advantage over conventional method of treatment. Information about their desorption characteristics of reuse, recovery of heavy metals and toxicity after treatment will allow their full scale application in industrial effluent and wastewater treatment. There is a vast scope to reconnoiter the feasibility of other materials such as molecular sieves, membranes, polymeric materials, microporous glass and carbon nanoscrolls etc. as adsorbents in regeneration, wastewater treatment, recovery of heavy metals and reuse.