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Synthesis and characterisation of stable and efficient nano zero valent iron

Abstract

Nano zero valent iron (nZVI) is an excellent adsorbent/reductant with wide applicability in remediation of persistent contaminants in soil, water and groundwater aquifers. There are concerns about its environmental fate, agglomeration, toxicity and stability in the air. Several modification methods have applied chistosan, green tea, carboxyl methyl cellulose and other coating substances to ensure production of nZVI with excellent air stability and effectiveness. The synthesis of a novel green nZVI (gNZVI) with Harpephyllum caffrum leaf extracts was successfully executed in the current study. Production of gNZVI involved the simultaneous addition of an optimum amount of the NaBH4 and H. caffrum extract to FeCl3 in an inert environment (Nitrogen). The solution was stirred for 30 min, washed with dilute ethanol (50%) and freeze dried. This procedure offered the best option for the synthesis of gNZVI in terms of nontoxic and inexpensive choice of stabiliser/reductant. Systematic characterisations using TGA, TEM, SEM, XRD, FT-IR and XPS confirmed the synthesis of crystalline, stable, reactive, well-dispersed and predominantly 50 nm diameter sized gNZVI compared to the conventionally synthesised nZVI which is 65 nm. The activity testing using Orange II sodium salt (OR2) confirmed the effectiveness of the synthesised gNZVI as an excellent Fenton catalyst with 65% degradation of 20 ppm OR2 dye in 1 h reaction time.

Introduction

One of the consequences of the current global upsurge in human population is massive pollution which leads to devastating infectious diseases and other problems. Contributions of human anthropogenic activities to environmental degradation and global warming are very obvious (He et al. 2016). Since the environment is heavily burdened by ever increasing anthropogenic activities, its protection and remediation should be prioritised. The most adversely contaminated sectors of the environment are air, soil and water. These three substances are an essential part of living systems and their importance to human survival is undeniable. Consequently, there is a need for the development of antipollution technology with significant capability for optimum performance. Nanotechnology has received considerable attention for various applications due its capability for manipulation of inherent desirable properties of materials such as reactivity, catalysis, strength, photo-activity and selectivity at the nano size (Cayuela et al. 2016; Sun et al. 2017). Environmental applications of nano materials include detection and elimination of pollutants in wastewater treatment plants as well as site remediation. The application of nano particles in the environment offers advantages such as improved performance, lower energy consumption and reduction in residual waste (Ali et al. 2008). Nano zero valent iron (nZVI) is one of the most researched and efficient nano materials for the mineralisation of a host of pollutants in contaminated water, soil and aquifers (Cao et al. 2005; Jewell and Wilson 2011; Yaacob et al. 2012; Bae et al. 2016; Lopez-Telleza et al. 2011; Pullin et al. 2017). nZVI is predominantly 10–80 nm sized form of iron particles with exceptional application due to their characteristic such as small size, large surface area, magnetism and oxidation state (Meral Turabik and Simsek 2017). Generally, iron-based materials are widely used in wastewater treatment, pharmaceuticals, medicine, food production and other manufacturing processes because of their low toxicity, biodegradability and cost effectiveness (Adeleye et al. 2016). There are possibilities of better and deeper delivery as well as higher reactivity in nano-sized substances rather than micro- or macro-scale particles (Xiu et al. 2010). Conversely, iron at an oxidation state of zero (Fe0) is usually unstable and readily returns to its natural state of electro-potential stability of Fe+2 or Fe+3 (Greenlee et al. 2012). The reactivity of nZVI depends on factors such as the source and nature of the iron material, manufacturing method, morphologies, nature of crystals, age of the nZVI and the presence of impurities (Pullin et al. 2017). The efficacy of nZVI against various types of microorganisms is well established (Xiu et al. 2010) and likewise its role in environmental remediation, treatment of contaminant in soil and water is also very popular (Kharisov et al. 2012). The integration of the unique properties nZVI in the development of sensor and diagnostic tools as well as biomedical application is a subject of intense investigations (Giersig 2008). In spite of its numerous applications and current commercial status, the universal acceptability of nZVI is hindered by its instability in moist environments. Apparently, the achievement of stability without compromising the reactivity of the nZVI and cost of production is a big challenge (Dong et al. 2016; Hannah G. Bulovsky 2016; Suponik et al. 2016).

Production of nZVI

Synthesis of nZVI can be achieved through the physical, physico-chemical or chemical reduction/modification of a higher oxidation state form of iron. The physical method which is a bottom-up approach includes nucleation from homogeneous solution, annealing at elevated temperature and separation of phases (Li et al. 2009). Meanwhile, physico-chemical methods such as reduction of goethite (α-FeOOH) or hematite (α-Fe2O3H) at an elevated temperature or the use of an ultrasonicator may also be involved in synthesis of nZVI (Jamei and Khosravi 2013). The products of these methods usually have a very high surface energy and, consequently, high agglomeration tendency (Mukherjee et al. 2016). Consequently, chemical synthesis is gaining popularity (Bae et al. 2016). It is a simple method which includes reductions of higher oxidation state of iron with active reducing agents such as sodium borohydride (Yuvakkumar et al. 2011). Usually, the produced nano iron has the propensity for high activity as a result of its larger surface area and intrinsic magnetic interaction. However, it is very unstable and rapidly reacts in air and moisture to give a non-homogeneous nano iron (Pullin et al. 2017). The high cost of chemical reducing agent, additional process of separation and large quantity of effluents are the limitations associated with industrial application of chemical synthesis. The ultimate method of synthesis must be cost effective, environmentally friendly and result in the production of nZVI with extensive application (Fazlzadeh et al. 2016; Wang et al. 2017). Various methods are already developed to ensure the synthesis of stable and effective nZVI for the purpose of environmental remediation and wastewater treatment, Table 1.

Table 1 Methods of nano zero valent iron synthesis

Characteristics of common modifiers

Surfactants

These are amphiphilic organic compounds which prevent agglomeration by steric or electrostatic repulsion. It is used for surface modification to improved solubilisation, desorption, transportation and reduction of agglomeration in an unstable nano particle (Gomes et al. 2014). The common examples are cationic surfactants such as cetylpyridinium chloride (CPC) and hexadecyl trimethyl ammonium (HDTMA) bromide or anionic surfactants such as sodium dodecyl sulphate (SDS) and sodium dodecyl benzene sulphonate (SDBS) as well as non-ionic surfactants such as polyoxyethylene alcohol ether, polyethylene, Saponin, Tween 80 and glycol octylphenol ether (Triton X-100). The uses of this type of modifiers are limited because of their high cost of production, inhibition of the activity of nZVI and the reversible nature of its surface.

Polymer

Surface modification with negatively charged non-organic polymer is possible due to the large molecular weight and high density of their charged functional group. It can enhance the dispersality of nZVI, increase its stability as well as reduction in its particle size. This type of modifier also has low/no environmental toxicity (He et al. 2007). The common examples are polyacrylamide (PAM), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP) and poly styrene sulphonate (PSS). They are also expensive, cause reduction in mobility of nZVI and consequently loss of efficiency.

Biopolymer

Like conventional polymer, naturally occurring, large molecular weight and high density substances can be used for surface modification of nZVI. The surface adsorption in biopolymer results in steric repulsion and increased viscosity of the suspension, and slow aggregation. The common examples are starch, guar gum (GG) and xanthan gum (XG). They are neutral, nontoxic, hydrophilic, stable, cost effective and biodegradable. The concentration of biopolymer must be determined before application as the excessive concentration may hinder the function of nZVI in subsurface environment (He and Zhao 2005).

Optimisation of nZVI is a subject of current intense research with significant achievements in the use of hydrophilic biopolymers (Jiao et al. 2015), carboxymethyl cellulose (Wang et al. 2010; He et al. 2007), chitosan (Geng et al. 2009), polyelectrolytes (Singh and Misra 2015), amphiphilic substances (Bishop et al. 2010) and various oil-based micro-emulsions (Berger et al. 2006). Currently, green synthesis remains the most outstanding method for the production of stable nZVI (Huang et al. 2014; Pattanayak and Nayak 2013). It involves the use of polyphenolic compounds from green plants (green tea, lemon, grape, balm, bran, sorghum, etc.) for chemical or physical synthesis of nZVI (Zaleska-Medynska et al. 2016). The priorities enjoyed by the polyphenolic containing green plants in the production of stable nZVI may due to their achievement of greater stability and effectiveness compared to the other methods (Mahmoud et al. 2016; Markova et al. 2014; Yew et al. 2016). However, the interaction between high oxidation state iron and polyphenolic plant extract cannot be clearly explained by a simple reduction reaction (Huang et al. 2014; Oakes 2013; Pattanayak and Nayak 2013; Yew et al. 2016). Besides, there are a lot of misconceptions about green chemistry and the nature of the association of polyphenolic constituent of green plants in the reduction of iron metal. The ideal capping agent for the efficient use of nZVI in environmental applications must be biodegradable to prevent further site contamination. It must not inhibit diffusion or adsorption of substrate to the active surface site. It must also be effective, stable and cost effective.

The stated properties of an ideal capping agent can also be found in some specific polyphenolic plant extracts which have been demonstrated to be capable of performing dual roles of reducing agent and capping agent in the production of nZVI (Mystrioti et al. 2014; Shahwan et al. 2011). A number of plant extracts from green tea, lemon balm, sorghum bran and weeds are widely reported to be rich sources of polyphenols (Oakes 2013). The choice of a polyphenolic plant candidate should be based on it abundance, biodegradability, solubility at room temperature and low market value of such plants. In these studies, the extract from the dried leaves of Harpephyllum caffrum, a common Southern Africa garden tree, was investigated for its ability to serve as reducing and capping agent in the synthesis of green nano zero valent iron.

$$ 4{Fe}^{3+}(s)+3{BH}_4^{-}(aq)+9{H}_2O(l)\to {Fe}^0\left(\mathrm{s}\right)+3{\mathrm{H}}_2{BO}_3^{-}(aq)+12{H}^{+}+6{\mathrm{H}}_2\left(\mathrm{g}\right) $$
(1)
$$ {Fe}^{2+}(s)+2{e}^{-}\to {Fe}^0\left(\mathrm{s}\right) $$
(2)
$$ {Fe}^0(s)+2{H}_2O\ (aq)\to {Fe}^{2+}\left(\mathrm{aq}\right)+{\mathrm{H}}_2(g)+2O{H}^{-}\left(\mathrm{aq}\right) $$
(3)

Wild plum (Harpephyllum caffrum)

The wild plum tree belongs to the family of Anacardiaceae (mango tree family). It is the fourth largest tree family in Southern Africa where it grows naturally in gardens. The tree is locally known as Umgwenya (Xhosa, Zulu), Mothekele (Northern Sotho), Wilde pruim (Afrikaans) or Wild Plum (English). The generic name, Harpephyllum, which means sickle-like was coined from the shape of the leaves. The edible fruit is commonly used for making jams and jellies while the bark and leaves have found use in traditional medicine (Olivier 2012). These are used for treatment of acne, eczema, skin rashes as well as blood purification among many other local uses. Alcoholic extract of the leaves of H. caffrum contains a high proportion of polyphenols which are documented to possess free radical quenching (Proestos et al. 2008), hepato-protection (Tian et al. 2012), anti-inflammatory and antimicrobial properties (Fratianni et al. 2014). The presence of polyphenolic compounds in the H. caffrum plant extract could hasten the reduction of Fe2+/Fe3+ into the desired zero oxidation form of nano iron as well as its stabilisation through the formation of a polyphenol Fe-complex (Oakes 2013). Such complexes have been thoroughly investigated and confirmed to have no ecotoxicological impact on living organisms (Markova et al. 2014; Rajan 2011). A number of polyphenolic compounds are already identified in the H. caffrum leaf (Nawwar et al. 2011), Fig. 1. Meanwhile, identification of reducing capability and confirmation of the polyphenolic constituent in H. caffrum leaf extract is necessary for the purpose of the current studies.

Fig. 1
figure1

Isolated polyphenolic compounds from the dried leaves of H. caffrum (Nawwar et al. 2011)

Experimental methods

Preparation of plant extract

Collection of the leaves of H. caffrum was done at the South African National Biodiversity Institute, Rhodes Drive, Newlands 7700, Cape Town, South Africa, on 17 June 2015. Authentication was performed by Anthony Hitchcock, nursery living collections and threatened species manager, Kirstenbosch National Biodiversity Garden. The polyphenolic compounds were extracted using the method of Sharma and Lall (2014) with little modifications. Shaded dried leaves of H. caffrum (40 g) were mechanically ground, soaked in 100 mL of aqueous ethanol (25%) and left on a shaker for 2 h. The extract was poured into a beaker and the extraction process was repeated until exhausted. The solvent was evaporated under reduced pressure and the resulting solution was freeze dried to give a brown amorphous powder which was stored in an air tight bottle at 20 °C.

Characterisation of plant extract

Antioxidant assay

The radical scavenging activity of the extracts was determined against the stable free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH). The assay involved reaction of DPPH with hydrogen donating antioxidant compounds in a reduction process that can be monitored by spectrophotometer at 517 nm as the DPPH colour changes from deep violet to light yellow (Brand-Williams et al. 1995). Of the H. caffrum extracts, 200 mg/L stock solutions were prepared by dissolving 10 mg of the dry extract in 50 mL of methanol. Further dilutions were made from the stock to get lower concentrations of the H. caffrum extracts (such as 100, 50, 25, 12.5 mg/L until 0.4 mg/L). Stock solution of ascorbic acid (standard reference) was also made (200 mg/L) and serially diluted to lower concentrations (such as 100, 50, 25, 12.5 mg/L until 0.4 mg/L). Blank test samples were prepared from 100 μL of serially diluted H. caffrum solution and 50 μL methanol while the negative control was 100 μL DPPH and 50 μL methanol. Fifty microlitres each of 150 mg/L DPPH in methanol was added to 100 μL of the serially diluted extracts, ascorbic acid, blanks and controls in a micro plate. The micro plate was subsequently kept in the dark for 30 min. The absorbance of the micro plate contents was detected using a Biotek Power-wave XS multi well reader (Analytical and Diagnostic Products, Johannesburg, South Africa). The experiments were run in triplicate and the values were converted into the percentage inhibition using the formula as given in the Eq. (4). The 50% inhibitory concentration (IC50) values were then calculated by linear regression of the plots using Graph Pad Prism version 5.

$$ \% Inhibition=\frac{\ \left(\ Abs.\kern0.5em Blank- Abs.\kern0.5em Sample\right)\ X\ 100}{Abs.\kern0.5em Blank\ } $$
(4)

Ferric reducing antioxidant power assay

Ferric reducing antioxidant power (FRAP) activity was measured according to the method developed by Benzie and Strain (1996) with little modifications. Briefly, acetate buffer (300 mM, pH 3.6), 10 Mm TPTZ (2,4,6-tripyridyl-s-triazine) in 0.1 M HCl and FeCl3·6H2O (20 mM) were mixed in the ratio of 10:1:1 to obtain the working FRAP reagent. Two hundred millilitres H. caffrum solution equal to 2 mg/mL was carefully prepared and agitated for 5 min with a Votex (Dragon LAB MX-S) and followed by centrifugation (Eppendorf centrifuge 5810R) at 1000 rpm for another 5 min to allow for formation of a clear solution of the test samples. One hundred millilitres of leaf extract of H. caffrum solutions was separately mixed with 300 mL of the prepared FRAP reagent. The sample absorbance was measured at 593 nm with the aid of “Multiskan” spectrum (Thermo Electro Corporation). Methanol solutions of FeSO4·7H2O ranging from 100 to 2000 μM were prepared and used for creating the calibration curve of known Fe2+ concentration. The parameter equivalent concentration was defined as the concentration of antioxidant having a Ferric-TPTZ reducing ability equivalent to that of 1 Mm FeSO4·7H2O.

Synthesis of green nano zero valent iron particles

The efficacy and stability of green nano zero valent iron depends on the amount of the nature and amount of the polyphenolic content as well as the reaction condition. In the current studies, the preparation of various forms of nZVI was as briefly described in Table 2.

Table 2 Nano zero valent iron samples and preparation conditions

FeCl3·6H2O (0.33 M) was prepared by dissolving 2.7 g of the hydrated iron III chloride hexahydrate (FeCl3·6H2O) in 50 mL 25% ethanol. The solution was purged with dried nitrogen gas for 2 min in a three-neck round-bottom flask to reduce the amount of dissolved oxygen. A 50 mL solution of NaBH4 (0.6 M) was prepared and put into a burette connected to a three-necked flask. The ferric solution was subsequently titrated with the NaBH4 solution at 20 °C. The reaction mixture was stirred for 30 min and harvested carefully by pouring through a vacuum filtration with a doubled 0.22 μm pore size cellulose acetate filter paper. The iron particles were subsequently washed several times with deionised (DI) water and ethanol mixture in an increasing amount of ethanol (100%) and later stored in a desiccator. The synthesised nano zero valent iron was labelled C. Meanwhile, in another instance, 200, 400 and 600 mg of dried leaf extract of H. caffrum were separately added to the mixture of 2.7 g hydrated iron III chloride hexahydrate (FeCl3·6H2O) in 50 mL 25% ethanol and 50 mL solution of NaBH4 (0.1 M) (as described above) to synthesise nano zero valent iron, labelled N, 4GN and 6GN, respectively. A third nano zero valent iron labelled G was also synthesised by addition of 1 g H. caffrum leaf extract to 2.7 g hydrated iron III chloride hexahydrate (FeCl3·6H2O) in 50 mL 25% ethanol and stirred for 12 h. The formed iron nano particles were separately harvested carefully by pouring through a vacuum filtration with a doubled 0.22 μm pore size cellulose acetate filter paper. They were subsequently washed several times with deionised water and ethanol mixture in an increasing amount of ethanol (100%) and later stored in a desiccator.

Activity test of nano zero valent Iron with Orange II sodium salt

The activities of differently synthesised nZVI (C, N and G) were tested by monitoring their catalytic roles in the degradation of Orange II sodium salt (OR2) (Fig. 2) during the Fenton processes. In this experiment, 200 mL solutions of 20 mg/L OR2 were prepared using deionised water. It is environmentally stable and can persist in the aqueous matrix. The decolourisation of this compound can be specifically monitored with the aid of a spectrophotometer at its wavelength of maximum absorption (483 nm). The prepared OR2 solutions were subjected to degradation by stirring at 500 rpm with 200 mg of the differently synthesised zero valent nano iron samples and 1 mL (105 mg/L) hydrogen peroxide. The samples were transferred into conical flasks and nZVI (C and N) separated at the end of 60 min by connecting a bar magnet to the round-bottom flask (magnetisation) while G treated (OR2) sample was filtered off because of its lack of magnetism. Finally, the degradation reaction of OR2 was quenched by increasing the pH of the treated OR2 solution to 10 using 1 M NaOH. A control experiment was set up without the zero valent nano iron. Another series of experiments were also setup with 200 mg of the differently synthesised zero valent nano iron samples at pH 5 to investigate the reduction capability of the synthesised nZVI. The synthesised zero valent nano iron samples were made to react with solution of 500 mL of 20 mg/L OR2 in nitrogen inert round-bottom flask.

Fig. 2
figure2

Keto tautomer of Orange II sodium salt

Result and discussion

Antioxidant activity

Following the extraction from the shaded dried leaves of H. caffrum, the antioxidant activity of leaf extract of H. caffrum was measured using simple, automated ferric reducing ability of plasma (FRAP) assay. The ferric was reduced to ferrous ion by polyphenolic substance at low pH causes a coloured ferrous-tripyridyltriazine complex to form. FRAP values were obtained by comparing the absorbance change at 593 nm in test reaction mixtures with those containing ferrous ions per dry weight of the tested sample as shown by Table 3.

Table 3 Antioxidant power of differently extracted polyphenolic compounds from the dried leaves of H. caffrum as recorded with the ferrous reduction antioxidant power (FRAP) assay

The FRAP of the extracts were 856.84, 863.30 and 1205.42 μmol/g for the ethanolic, (50%) aqueous-ethanolic and aqueous extract, respectively (Table 3). The result shows that the aqueous extract has the highest power for the reduction of free radicals, i.e. changing the higher oxidation state of iron to a lower one. Consequently, a smaller amount of the aqueous extract will be needed for the reduction of a higher oxidation state iron to a lower one compared to the ethanolic and (50%) aqueous-ethanolic extracts. Since the knowledge of specificity in metal reduction by the polyphenolic constituents of H. caffrum is yet to be confirmed, it is therefore important to use at least 20% ethanol as the extraction solvent. Besides, the current FRAP results is a confirmation of the findings by Oakes (2013) that the H. caffrum leaf extracts contain a high amount of polyphenolic compounds with capability to act as a reducing agent by donating electron for the reduction of Fe3+. On this note, it is expected that compounds that donated electrons to reduce Fe3+ to Fe2+ can equally donate electrons to quench free radicals. Apparently, radical scavenging activity of the extracts was determined against the stable free radical DPPH, Table 4. The focus of the next phase of the studies was therefore directed to DPPH antioxidant activities of aqueous-methanolic (4:1) dried leaf extracts of H. caffrum.

Table 4 Antioxidant activities of aqueous-methanolic (4:1) dried leaf extracts of H. caffrum as recorded from the DPPH assay

H. caffrum showed a high percentage inhibition (more than 80%) for DPPH at 100 mg/L. However, the used reference standard reductant, i.e. ascorbic acid, has a significantly higher percentage inhibition (more than 97%) as shown in Table 4. Consequently, H. caffrum can significantly react in a biological matrix and inhibit oxidative processes (Nawwar et al. 2011). Meanwhile, the concentration of the ascorbic acid and H. caffrum needed for the half maximum oxidation of DPPH (IC50) were 7.94 and 39.10 mg/L, respectively, as calculated from the plot of % inhibition versus the log of antioxidant concentration (Fig. 3). Although, H. caffrum leaf extract shows high antioxidant capability but it was comparatively lower than that of the standard reference (ascorbic acid). According to the current findings, 4.70 multiple of H. caffrum (IC50 = 38.02 mg/L) can effectively inhibit oxidative process just like a unit of ascorbic acid (IC50 = 8.13 mg/L). The disparity in the IC50 of H. caffrum leaf extract and that of the standard (ascorbic acid) is apparently due to the high quantity of non reactive contents in H. caffrum.

Fig. 3
figure3

Correlation graph of % inhibition and log of antioxidant concentration

Identification of polyphenolic compounds in the leaf extracts of wild plum

The reported antioxidant activities of leaf extracts of H. caffrum may be due to its polyphenolic content. This was confirmed by the FT-IR analysis of the ethanolic-aqueous extract (1:4) using Perkin Elmer PE1600. The FT-IR results were able to confirm the presence of polyphenolic compounds in H. caffrum (Fig. 4). Peaks around 1437 and 1603 cm−1 are ascribed to aromatic skeletal vibration and carboxylic groups respectively (typical of phenolic compounds) while 2919 and 3267 cm−1 are assigned to C-H stretching and O-H stretching.

Fig. 4
figure4

FT-IR spectroscopy of aqueous-ethanolic extract (4:1) of H. caffrum

Furthermore, liquid chromatography-mass spectroscopy (LC-MS) analysis was done in other to confirm the specific polyphenolic compounds in the studied H. caffrum plant extracts (Figs. 5 and 6).

Fig. 5
figure5

LC-MS Analysis of ethanolic extract of H. caffrum identifying Kaempferol 3-O-galactoside, Kaempferol 3-O-rhaminoside and Quercetin-3-O-arabinoside at 431.2, 447.1 and 417.1, respectively

Fig. 6
figure6

LC-MS analysis of aqueous-ethanolic extract of H. caffrum identifying Quercetin and Kaempferol at 303.1 and 287.1, respectively

Five of the previously isolated polyphenolic compounds (Nawwar et al. 2011) were confirmed in the aqueous-ethanolic extract of H. caffrum during the current studies using GC-MS. These compounds are Quercetin, Kaempferol, Kaempferol 3-O-galactoside, Kaempferol 3-O-rhaminoside and Quercetin-3-O-arabinoside. Quercetin and kaempferol are flavonols while the others are their sugar based derivates. Each of these compounds can participate actively in the reduction of biological cells as well as elemental iron (Sharma and Lall 2014). The compounds are polyphenols with the typical characteristic (catechol and gallol) functionalities and metal chelating. They bind with iron to form octahedral geometry (mostly in the ratio 3:1). The binding involves several equilibrium constants with the resultant high overall complex’s stability constant (Perron and Brumaghim 2009). The deprotonated polyphenolic compound generates “hard-ligands” which possess an oxygen centre with a high charge density. This hard-ligand is a form of strong Lewis base which forms stable complex with a Lewis acid such as Fe3+ (Texas 2016). The reaction is specific for certain types of metal and depends on oxidation state and medium pH (Wang et al. 2017). The contribution of sugar-based derivatives of polyphenolic compound in this synthesis cannot be overlooked. This is well documented by Demir et al. (2013) in the synthesis and capping of super paramagnetic FeSO4 nano particle through the reduction process by maltose and its glucose derivatives. Likewise, glucose has been used as a reducing agent in the green and facile synthesis of super paramagnetic Fe3O4 nanoparticle while gluconic acid stabilises and disperses the produced nano iron (Lu et al. 2010).

It is now very clear that polyphenolic compounds easily give up its hydrogen in contact with a higher oxidation state metal and subsequently form a metal-polyphenol complex (Mystrioti et al. 2014; Yew et al. 2016). Consequently, polyphenolic constituents of green plants can be responsible for reducing, dispersing and capping of the iron-polyphenol nanoparticles through complexation processes (Wang et al. 2017). Iron-polyphenol complex nano particle structure has been proposed due to partial reduction of Fe(3+) and subsequent auto-oxidation (Saif et al. 2016; Stefaniuk et al. 2016). These processes may involve oxidation-reduction reactions followed by oligomerisation. The stable and surface modified nZVI as described in these studies (Fig. 7) may be easily agglomerated and consequently become less active iron nano particle. Apparently, iron-polyphenolic complex is too stable to be reactive and since the target of this study is to synthesis a stable but reactive nZVI, it is therefore necessary to optimise the amount of polyphenolic compound that can be used in the synthesis of nZVI for good stability and reactivity.

Fig. 7
figure7

Theoretical structure of quercetin-zero valent iron complex

Optimisation of polyphenolic compounds load

nZVI “N” was synthesised by adding 200 mg dried leaves extract of H. caffrum (20 mL) to FeCl3 during its reduction with a small amount of sodium borohydride (200 mg) while 4GN and 6GN were synthesised by separate addition of 400 and 600 mg dried leaves extract of H. caffrum to FeCl3 sodium borohydride (200 mg). The determination of thermal stability of the particles as well as the compositions of the polymeric blends of plant extract can be done using thermogravimetric analysis (TGA) of the differently synthesised nZVI (Perkin Elma STA 4000) (Fig. 8).

Fig. 8
figure8

TGA of synthesised Fe0 particles showing how varied amounts of H. caffrum extract affect stability of the nano zero valent iron

In the TGA analysis, N undergo slow mass loss and retain about 65% metal residue while 4GN and 6GN undergo rapid mass loss and retain 30 and 20% metal residues, respectively. The implication of these results is that N is the most stable of the three samples with good amount of residual nZVI and weight loss less than 30% at 500 °C. Apparently, 200 mg H. caffrum is appropriate as reductant/surface modifier in the preparation of nZVI with impressive thermal stability.

Characterisation of the synthesised nano zero valent iron

It is very important to compare the nZVI G, synthesised using only H. caffrum plant extract (1 g), the nZVI N, synthesised with both sodium borohydride (500 mg) and H. caffrum plant extract (200 mg), with conventional nZVI C, synthesised using sodium borohydride (1 g), by studying their characteristics using common analytical equipment such as X-ray diffraction (XRD), TEM, SEM, FT-IR, X-ray photoelectron spectroscopy (XPS) and TGA. Finger print identification of the synthesised nZVI can be analysed using XRD spectra. Figure 9 is the XRD pattern for the synthesised nano zero valent irons.

Fig. 9
figure9

X-ray diffraction (XRD) pattern of C, N and G

A broad peaks at 44.6° of 2θ presented in this study is a confirmation of the presence of nano zero valent iron in its crystalline form as described by Taha and Ibrahim (2014). This is in accordance with diffraction pattern of body-centered cubic α-Fe (JCPDS No. 06-0696). Both the conventionally synthesised nano zero valent iron, C, and the nano zero valent iron modified by simultaneous addition of H. caffrum extract, N, have identical XRD spectra patterns with peaks as described above. However, the appearance of small peaks around 22.7° and 63.2° on close inspection of the XRD spectrum of C is a confirmation of the presence of ϒ-FeOOH (Lepidrocite) phase (JCPDS No. 17-0536). This is as a consequence of the oxidation of nZVI in moist air due to its instability and absence of encapsulation or surface protecting substance. Therefore, C is an “iron-zero” core surrounded by higher oxidation state iron oxides (Fe2+/Fe3+). The result is in agreement with previously publications on nano zero valent iron (Hoag et al. 2009; Kozma et al. 2016; Liu and Zhang 2014). Besides, XRD spectra of nano zero valent synthesised with the extract from H. caffrum (G) lack the typical well-defined peaks. This is an indication of its amorphous nature. The nano-sized crystal substances scatter the X-ray in many directions leading to a broader peak. Both the encapsulation and surface oxidation of nZVI can alter its surface functionalities and increases its size. The determination of the surface functionality and size of nZVI can be better done with TEM and SEM analyses.

High resolution transmission electron microscopy (HRTEM) image of nZVI C, N and G are as presented in Figs. 10, 11 and 12, respectively. It was observed through the TEM images that the nZVI C is a crystalline form of zero valent iron (Fe0) with a number of agglomerated particles while N is a relatively well-dispersed crystal of zero valent iron and G had non distinct particles, making the determination of its size difficult. Sizes of the synthesised nano particles are in concordance with the previously published report on the nZVI (Markova et al. 2014; Thomé et al. 2015; Yaacob et al. 2012). C is predominantly 60 nm in diameter while N is 50 nm, thus N has a larger surface area compared to the conventionally synthesised nZVI C. Besides, the surface morphology of conventionally synthesised nano zero valent iron C, as revealed by SEM analysis, shows the chain-like agglomeration of particles in which several layers of small particles clump together to form bigger agglomerates SEM analysis of N shows a better-dispersed spherical shaped nano particles.

Fig. 10
figure10

TEM of C (scale, 100 nm) particle size distribution and the particle size bar chart

Fig. 11
figure11

TEM of N (scale, 200 nm) particle size distribution and the particle size bar chart

Fig. 12
figure12

TEM image of G showing the granular particle

The differences in the structural bonds and intermolecular forces are responsible for surface morphological differences exhibited in the synthesised nano zero valent irons. This can be investigated by FT-IR analysis results (Fig. 13).

Fig. 13
figure13

FT-IR spectra of H. caffrum (HF) and synthesised nano zero valent irons G, N and C

There are broad OH stretching vibration of around 3282 cm−1 on the FT-IR spectra of C, G and HC. The OH spectrum in the FT-IR of C is due to the formation of (ϒ-FeOOH) Lepidrocite on the surface of nZVI. The result is a confirmation of the claim that the surface of the synthesised nZVI is predominantly iron at higher oxidation state (Ashokkumar and Ramaswamy 2014; Liu and Zhang 2014). Besides, the spectra of G and HC also show intense broad OH stretching vibrations due to the contribution of benzylic OH from the polyphenolic plant extract. Meanwhile, the peak due to the aromatic and moisture from air is present but not prominent in the spectra of nZVI modified with H. caffrum extract N. The intense aromatic skeletal vibration peaks around 1598 cm−1 in the FT-IR spectra of HC and G suggested the binding of the polyphenolic groups to the active surfaces of the synthesised nano iron particles. These peaks suggest the presence of aromatic groups in the compounds while its high intensity in G is a confirmation of existence of Fe-O complex as previously documented by Lu et al. (2010) and Wang (2013). Similarly, symmetric and asymmetric carbonyl peaks at 1218 and 1379 cm−1 as well as the observed C-O stretching peak at 1042 cm−1 are present on the spectra of HC, G and N. There are specific distinctions between H. caffrum (polyphenol) containing nano iron and the one synthesised with only NaBH4 (C). The carbonyl containing sugar base in H. caffrum serves as surface modifier in the novel nanoparticle G and N; hence, it prevents agglomeration and enhances the activity in nZVI (N). This is possible through the polymer entanglement and hydrogen bonding of the sugar moiety of the polyphenolic plant extract (Lu et al. 2010; Thekkae et al. 2017). However, the amount of H. caffrum needed for the stability of nZVI must be optimised to prevent possible loss of its activity.

Stability of the synthesised nano zero valent iron

Thermogravimetric analysis can be used to measure the amount and rate of change in weight of nZVI material as a function of temperature in a controlled atmosphere. In these studies, the thermal stabilities of synthesised nZVI as revealed by TGA (Fig. 14) were used to estimate the amount of nano iron present in a unit of nZVI as well as speculating on their stability in the environment.

Fig. 14
figure14

TGA graph (triplicate) of differently synthesised nano zero valent iron at optimised conditions

Thermal profiles of the synthesised nZVI show that the TGA analysis of the conventional nano zero valent iron “C” (synthesised using sodium borohydride) has a low percentage weight loss against the operating temperature. Consequently, C has a high residual iron base (more than 80%) because of the fact that no plant extract was used in its synthesis. Similarly, nano zero valent iron “N” which contains optimised amount of H. caffrum also has high residual iron base (more than 70%) and shows uniform loss of volatilised polyphenolic compounds across the analysis temperature (0–900 °C). Meanwhile, nano zero valent iron “G” which was synthesised with only H. caffrum extract has a low residual iron base (about 25%). The low percentage iron residual in G may be caused by slow exchange of valent electrons between Fe and the polyphenolic constituent of H. caffrum extract which results in formation of stable iron complex. The 25% constitution of Fe in nano zero valent iron G is in support of the theoretical structure of Quercetin-zero valent iron complex that was speculated earlier (Fig. 7). The amount of iron particles and its environmental stability is very important to its activity during the application. However, nano iron stability and reactivity are highly dependent on the oxidation state of its surface.

Oxidation of the synthesised nano zero valent iron

Optimisation of material may not be possible without good understanding of both the physical and chemical interactions at its surface or the interface of its layers. The chemical states of nZVI particles surface were investigated by a high resolution XPS. Both elemental composition and empirical formula of a material can be measured using XPS. The spectra are obtained by irradiating a solid surface with a beam of X-ray while the kinetic energy and emitted electron are simultaneously measured (Figs. 15, 16, 17).

Fig. 15
figure15

XPS spectra indicating Fe 2p3/2 peaks of the nano zero valent irons G, N and C

Fig. 16
figure16

XPS spectra indicating carbon peaks in the synthesised nano zero valent irons G, N and C

Fig. 17
figure17

XPS spectra of 1s oxygen peaks in the synthesised nano zero valent irons G, N and C

The presence of photoelectron peaks on the XPS spectra of C, N and G at ~ 707, ~ 709 and ~ 710 (Fig. 15) represents the presence of iron in a zero valent state and its oxidised forms in the structure of the synthesised nZVI. The absence of 2p3/2 and 2p1/2 at ~ 723 eV BE in the spectrum of nZVI G is an indication of its relative stability in moist air, i.e. no higher oxidation state iron (Fe2+, Fe3+) is present on its surface. Meanwhile, the conventionally synthesised nZVI C and the modified N are core zero oxidation state irons surrounded by their oxidised forms. These findings were supported by Mu et al. (2017) among many other previously published articles on nZVI. The presence of adventitious carbon was shown by the spectra of C at ~ 284 eV BE (Fig. 12); meanwhile, the peaks which were more intense (at ~ 284 eV BE) represent the carbon-rich polyphenolic compounds on the surface of N and G as supported by Yan (2011). Also presented on the XPS spectra are the oxygen 1s peaks at ~ 530 and ~ 531 eV BE. The binding energy of 1 s peaks for G was comparatively higher (~ 531 eV) because of the extensive conjugation between the iron and the polyphenolic group in the H. caffrum.

Application of the synthesised nZVI using Orange II sodium salt

The use of nZVI as a catalyst in the oxidation of organic dye by hydrogen peroxide during the treatment of wastewater from dye textile or manufacturing companies is very common. And likewise, the acidic pH which was ranged between 2 and 4 was widely reported for Fenton oxidation of nZVI.

Following the degradation of 10 ppm Orange II sodium salt solution with the conventionally synthesised (with NaBH4) nZVI C, the optimum degradation condition (64% per hour) was achieved at an acidic pH of 2 and there was a reduction in reactivity of nZVI as the pH of the initial concentration increased (Fig. 18). The results show that nZVI reacts in a Fenton-like process in which the optimum pH condition is acidic. Samaei et al. (2015) also reported the same acidic condition at pH 3.5 as their optimum for the degradation of methyl tertiary butyl ether using 10 ml/L H2O2 and 0.25 g/L nZVI. Meanwhile, Chahbane et al. (2006) has justified the possibility of nZVI participating in reduction, oxidation, adsorption and catalysis reaction.

Fig. 18
figure18

pH-dependent activity of nano zero valent iron “C” at 20 °C, 20 ppm OR2, 500 rps

The catalytic activities of C, N and G were compared in Fenton oxidation by applying nZVI samples (200 mg) to a solution of 1 mL (105 ppm) hydrogen peroxide and OR2. N demonstrated the best activity, by achieving 65% degradation of OR2 (20 ppm, 500 mL) per 1 h of reaction time (Fig. 19). This was followed by C (59%) while the least reactive of nZVI as previously speculated is G (26%). The H. caffrum extract is suitable for surface modification of nano zero valent iron for enhanced air stability and significant increases in activity due to reduction in agglomeration and increased surface area. The nZVI G, which was prepared using H. caffrum without NaBH4, was too stable to react spontaneously like the modified nano iron N because of the strong intermolecular bond between the polyphenolic compound and iron which render the surface of G less active.

Fig. 19
figure19

UV spectra showing change in absorbance of orange II sodium salt (20 ppm, pH 2, 20 °C) using the synthesised nZVI as a Fenton catalyst with the inserted % degradation bar chart

The reducing property of the synthesised nZVI (200 mg) was also investigated by using 20 ppm OR2 at pH 5 and 20 °C under anoxic condition. OR2 undergoes significant degradation during the reduction reaction by the nano particles (Fig. 20). The most reactive of nZVI remains N followed by C with obvious degradation of the azo linkage of OR2. G is the least active of the nano particles, behaving much more like an adsorbent than a reductant. The synthesised nano particles have capabilities has reductant, catalyst and adsorbent. The mechanisms of their reactions were previously reported (Kim et al. 2015; Lopez-Telleza et al. 2011).

Fig. 20
figure20

UV spectra of orange II sodium salt degradation (20 ppm, pH 5, 20 °C) using the synthesised nZVI as reductants

Conclusion

The synthesis of an efficient and stable novel green nano zero valent iron (N) was achieved through the simultaneous addition of the leaf extracts of H. caffrum and NaBH4 to the FeCl3. H. caffrum, its high antioxidant activity and polyphenolic content acts as capping and reducing agent and ensures the stability of nZVI in air and moisture. The synthesised nZVI (N) possessed a significant higher surface area in comparison with the conventionally synthesised nZVI (C) and nZVI obtained through the leaf extracts of H. caffrum reduction of FeCl3. Its potential as an effective Fenton catalyst was confirmed by the comparatively high decolorisation of tested Orange II sodium salt dye pollutant. The prominent reactive polyphenolic substances in the leaf extract of H. caffrum exhibited the capability for reduction of high oxidation state iron to nano zero valent iron. At the same time, formed metal-organic organic complexes which protect the nano zero valent iron against oxidation in moist air and thereby enhance stability. The use of locally sourced environmentally benign and renewable material as described in these studies will enhance economic competiveness and sustainability in the production of nano materials. Besides, the successful synthesis and characterisation of nZVI G through the reduction of FeCl3 by the leaf extracts of H. caffrum has settled the misconception relating to the use of plant extracts in nano synthesis. Although, the synthesised green nZVI G had less activity as a Fenton catalyst, its prospect as an excellent adsorbent with longer shelf life is proposed by it characterisation and will be established in future studies.

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Badmus, K.O., Coetsee-Hugo, E., Swart, H. et al. Synthesis and characterisation of stable and efficient nano zero valent iron. Environ Sci Pollut Res 25, 23667–23684 (2018). https://doi.org/10.1007/s11356-018-2119-7

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Keywords

  • Nano particle
  • Polyphenolic
  • Pollutants
  • Chistosan
  • Harpephyllum caffrum