Keywords

1 Introduction

Environmental sustainability is defined as responsible interaction with the environment to avoid depletion or degradation of natural resources and allow for long term environmental quality. However the world’s definition of sustainability is sustainable development which results in environmental degradation. The advancement in science and technology contribute directly or indirectly to the increase in waste and toxic materials in the environment. Environmental sustainability programs include protection and restoration of the natural environment.

One of the restoration strategies used nowadays is bioremediation which makes use of microorganisms. The advantages of bioremediation over conventional treatments is cost effectiveness, high competence, minimization of chemical and biological sludge, selectivity to specific metals, no supplementary nutrient requirements, regeneration of biosorbent, and the possibility of metal recovery (Kratochvil and Volesky 1998). However this treatment method is not feasible for sites contaminated with toxic substances as it is harmful to microorganisms.

Nanoparticles however have the unique capability to remediate such toxic environments and also provide a healthy substrate for microbial activity thus speeding up the process of environment clean-up. Nanoparticles can be prepared by physico-chemical methods (Masala and Seshadri 2004; Swihart 2003) but the use of hazardous chemicals, high cost and toxic byproducts has given biological nanoparticle synthesis an advantage (Konishi et al. 2006). Nanobioremediation is the use of nanoparticles to remove pollutants by enhancing microbial activity.

Nanoparticles (NPs) may be either metallic or nonmetallic and differently shaped. NPs are of the following types-single metal NPs, bimetallic NPs, carbon based NPs, modified NPs etc. Metal nanoparticles have applications in different fields like medical imaging (Lee et al. 2008a), drug delivery (Horcajada et al. 2008), electronics (Lipovsky et al. 2008), nanocomposites (Seager et al. 2007), biolabeling (Liang et al. 2006), biocide or antimicrobial agents (Sanpui et al. 2008), sensors (Jiang et al. 2008), non-linear optics (Ebothe et al. 2006), hyperthermia of tumors (Pissuwan et al. 2006), intercalation materials for electrical batteries (Klaus-Joerger et al. 2001), optical receptors (Dahan et al. 2003) etc. The wide range of applications of nanoparticles is due to their unique optical, thermal, electrical, chemical, and physical properties (Panigrahi et al. 2004).

Nanoscale materials has the following characteristics—Larger surface area per unit mass, show quantum effect and hence is more reactive, exhibit plasmon resonance and can diffuse or penetrate contaminated sites easily. Selective sample extraction can be achieved by modifying the surface functionality of the nanoparticles.

In this review we have focused on the biosynthesis of five nanoparticles (Zn, Ag, Au, Fe, Cu) using plant extracts, bacteria, fungi and algae. We have also compiled the different degrading and remediation activities of these nanoparticles for possible large scale restoration programs.

2 Biosynthesis of Zinc Nanoparticles (ZnNP’s)

In the last two decades ZnNP’s have been given significant attention owing to its applications in varied fields like piezoelectric films (Martin et al. 2000), piezoelectric sensor (Wang et al. 2004), ceramics (Grigorjeva et al. 2008), photo catalysis (Pal and Sharon 2002) solar cells (Gordillo 2002), bio-imaging, drug delivery (Xiong 2013) actuator, biosensors (Yang et al. 2012) and water purification specifically arsenic removal (Singh et al. 2013b).

Owing to the wide applications of ZnNP’s it has been synthesized by methods such as wet chemical method (Lee et al. 2009; Mehta et al. 2012), organic solvent method (Mezni et al. 2012) and microwave method (Nehru et al. 2012). However biological synthesis using bacteria, fungi, algae and plant extracts is the method of choice and many researchers have been successful in synthesizing ZnNPs (Table 1).

Table 1 Biosynthesis of ZnNPs

3 Biosynthesis of Silver Nanoparticles (AgNP’s)

Nanobiotechnological developments have led to the development of environmentally benign nanoparticles. AgNP’s have applications in non-linear optics, as intercalation materials for electrical batteries, optical receptors, as a catalyst and as an antibacterial. The antimicrobial activity of silver nanoparticles has many uses like the production of AgNP coated blood collecting vessels, coated capsules, bandaids etc. (Geoprincy et al. 2011). Biosynthesis of AgNp’s using biological entities is tabulated in Table 2.

Table 2 Biosynthesis of AgNP’s

4 Biosynthesis of Gold Nanoparticles (AuNP’s)

Gold nanoparticles show high chemical reactivity on comparison with bulk gold. It exhibits surface plasm on oscillations which can be used in fields like labeling, imaging and sensing. AuNPs are biocompatible and hence can be used in disease diagnosis and therapy. AuNPs have been biosynthesized from plant extracts, bacteria, fungi and algae. Table 3 focuses on the biological synthesis of AuNP’s.

Table 3 Biosynthesis of AuNP’s

5 Synthesis of Iron Nanoparticles (FeNP’s)

The three major forms of iron oxides found in nature are magnetite (Fe3O4), maghemite (γ-Fe2O3), and hematite (α-Fe2O3) (Cornel and Schwertmann 1996). Of all the different kinds of iron oxides, magnetite have aroused the interest of many researchers owing to the fact that it can be easily synthesised, can be modified or coated and has superparamagnetic characteristics (McHenry and Laughlin 2000). This property makes it easy to separate these supermagnetic particles from aqueous solution and complicated matrices by applying an external magnetic field. Some problems associated with using FeNP’s are its intrinsic instability resulting in the formation of agglomerates and its high chemical activity which promotes oxidation, subsequent loss of magnetism and dispersion. Hence for application in various fields these nanoparticles have to be coated with either inorganic substances like silica, carbon etc., or with organic species like surfactants and polymers (Wu et al. 2013). Iron nanoparticles have been synthesized by various chemical and physical methods (Afonso et al. 2011). Some emerging methods of synthesis of iron nanoparticles are the use of microorganisms and plant extracts which exclude the use of harmful chemicals and toxic byproducts (Table 4).

Table 4 Biosynthesis of FeNP’s

6 Synthesis of Copper Nanoparticles (CuNP’s)

Copper is one of the most widely used materials in the world owing to its usage in fields like electricity, optics, catalysis, biomedical and antimicrobial applications. Many researchers have been successful in the biosynthesis of copper nanoparticles using the seed, flower, leaves and fruit skin of plants (Table 5). The nanoparticles synthesized from plant extracts were found to be covered by the medicinal properties of the plant. CuNP is an antimicrobial agent used in food packaging and water treatment.

Table 5 Biosynthesis of CuNP’s

7 Nano Bioremediation

Population growth, rapid industrialization and long term droughts has resulted in the spread of wide range of pollutants in surface and ground water system (Chong et al. 2010). The major contaminants include heavy metals, inorganic compounds, organic pollutants and many other complex compounds (Li et al. 2011). It is imperative to remove these toxic substances as they are harmful not only to human beings but also to the ecological environment (Pang et al. 2011). Waste water treatment processes like photo catalytic oxidation, adsorption/separation processing and bioremediation (Huang et al. 2006; Zelmanov and Semiat 2008) have been tried and tested. But factors like efficiency, operational method, energy requirements and high cost have restricted their usability (Huang et al. 2006; Zelmanov and Semiat 2008). In the past two decades nano scale materials have been used as an alternative to existing treatment materials due to its efficiency, cost effectiveness and eco-friendly nature (Dastjerdi and Montazer 2010).

Iron NPs is considered to be the first nanoparticle to be used in environmental clean-up (Tratnyek and Johnson 2006). Current applications of iron-based technologies in contaminated land or groundwater remediation can be broadly divided into two groups, based on the chemistry involved in the remediation process: technologies which use iron as a sorbent (adsorptive/immobilization technologies) and as an electron donor to break down or to convert contaminants into a less toxic or mobile form (reductive technologies) (Cundy et al. 2008). However, it should be noted that many technologies utilize both these processes.

Zn NPs a semiconductor photo catalyst have been extensively studied by researchers around the globe owing to its capacity to degrade organic dyes. ZnNPs can photo catalyse and cause the complete degradation of a wide variety of compounds from dyes to phenols and pharmaceutical drugs (El-Kemary et al. 2010).

Among all nanoparticles noble metal nanoparticles like gold and silver have enormous applications in diverse areas. In recent times researchers have analyzed the potential of Au and Ag nanoparticles in the degradation of organic dyes. Copper nanoparticles also can be used in the degradation of organic dyes with good results. Table 6 focusses on the degradation and subsequent remediation of various environmental pollutants.

Table 6 Remediation by metal nanoparticles

8 Conclusions

Nanotechnology is revolutionizing the way we live. The unique characteristics of nanoparticles have made them the particle of choice in many fields including remediation of environmental pollutants. Ecofriendly synthesis of nanoparticles coupled with remediation can go a long way in promoting sustainability. Biosynthesis helps minimize the use of harmful chemicals and solvents and is simple, cost effective and time saving. Zn, Ag, Au, Fe and Cu nanoparticles have been synthesized by many researchers using various biological methods. Although there are many studies on the synthesis of Au, Ag, Cu and Zn nanoparticles, very few have concentrated on the biosynthesis of FeNPs using bacteria, fungi and algae. Green synthesis of metal NPs using plant extract seems to be the subject of choice of a majority of researchers, however only a few is documented in this review. On comparing the bioremediation properties of these NPs it was noted that FeNPs has a wider application, degrading pollutants like pesticides, dyes, hydrocarbons, TCE etc. Most of the other NPs find applications in the photocatalytic degradation of dyes. Considering the wide applications of FeNPs in remediation it is necessary to find novel methods to biosynthesize FeNPs on a large scale.