Eco-friendly synthesis and characterization of phytogenic zero-valent iron nanoparticles for efficient removal of Cr(VI) from contaminated water
Zero-valent iron nanoparticles (ZVIN) are widely synthesized by several methods in the last decades because it offers indisputable advantages to almost every area of expertise, heavy metal ions removal, environmental remediation including for the wastewater treatment. Herein, we report for the first time, the green and eco-friendly synthesis of phytogenic ZVIN using reproducible Catharanthus roseus (CR) flower extract for the removal of heavy metal ions including Cr(VI) by adsorption isotherms. The synthesized stable ZVIN were characterized by UV-visible absorption spectroscopy (UV-vis), Fourier-transform infrared (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). The FT-IR analysis reveals that the polyphenolic compounds that are present in the CR flower extract may be responsible for the reduction and stabilization of the ZVIN. The XRD and SEM-EDX analyses confirmed the phase, composition of elements and morphology of the ZVIN. The synthesized low-cost and non-toxic ZVIN used for the adsorption removal of Cr(VI) from contaminated water; and the Langmuir and Freundlich adsorption isotherms are used to study the adsorption process by the experimental equilibrium adsorption data. The maximum removal of Cr(VI) (98.28%) was observed using optimal conditions of 1.6 g/L of ZVIN concentration, 10 ppm of Cr(VI) concentration, and pH = 4.3 of the initial solution. The adsorption removal of Cr(VI) using the synthesized ZVIN as follows pseudo-second-order kinetic equation with a corresponding correlation coefficient of (R2 = 0.99).
KeywordsZVIN Catharanthus roseus flower extract Cr(VI) removal Adsorption isotherms Kinetics
Contamination of air, water, and soil by toxic heavy metal ions is one of the serious environmental problems faced by the world today. Heavy metals represent a group of dangerous environmental pollutants and due to toxic effects on human health in concentrations above the permissible limits, cause widespread concerns . Non-degradable heavy metals also can accumulate in living tissues and enter into the food chain which causes huge damage to the biosphere in the form of various diseases and disorders [1, 2, 3, 4, 5]. Among all, hexavalent chromium (Cr(VI)) is one of the most hazardous heavy metal ions in nature. With the industrialization, a large quantity of wastewater containing high concentrations of Cr(VI) is released into the environment from distinct industrial activities, including iron and steel manufacturing, electroplating, metal cleaning, preservation of food, leather tanning, pigment production, and other anthropogenic sources [2, 3, 4, 5]. The Cr(VI) metal ion is highly toxic to human and animal tissues because of its potential carcinogenic, teratogenic, and mutagenic properties [5, 6, 7, 8]. In general, Cr(VI) may be in the form of negatively charged oxyanions such as dichromate (Cr2O7−2) or chromate (CrO4−2) or hydrogen chromate (HCrO4−) in solutions. As highly soluble Cr(VI) is freely transferred in aqueous environments due to repulsive electrostatic interactions among anionic Cr(VI) species and negatively charged particles of soil, it is necessary to remove Cr(VI) from industrial wastewater before entering into the environment.
Several methods are investigated for the removal of heavy metals such as filtration, ion exchange, precipitation, membrane process, sedimentation, electrochemical treatment, reverse osmosis, photocatalytic reduction, and adsorption [9, 10, 11, 12]. Out of these, adsorption is considered more preferable for eliminating heavy metals due to its unique properties like ease of operation, the simplicity of design, insensitivity to toxic pollutants, fewer amounts of harmful substances, and low cost [13, 14]. There are many eco-friendly bioadsorbents, but those show poor adsorption capacity and very slow process kinetics. But nowadays, nanoscale zero-valent iron is widely used for the adsorptive removal of heavy metals due to its large specific surface area, high reactivity of surface sites, small particle size, and strong reducibility [15, 16].
Mostly, ZVIN are synthesized by a liquid-phase reduction method using sodium borohydride or potassium borohydride as reducing agents . But the main disadvantage of this method is toxicity of borohydride which eventually may induce environmental pollution in the form of resulting ZVIN. Furthermore, ZVIN prepared by this method can agglomerate quickly in clusters. Hence, to prevent agglomeration process, it is required to add some dispersant agents like stabilizing or capping agents [18, 19]. If non-biodegradable dispersant agents used in large scale, then those are treated as secondary pollutants. Therefore, there is a clear need for novel alternative method which obeys the tenets of green chemistry, for the preparation of ZVIN. In the process of green chemistry, the plant extract mediated synthesis which is economically preferable and environmentally compatible has been proposed as a preparative method for ZVIN [20, 21]. Naturally, plant extracts contain polyphenols, flavonoids, and other phytochemicals that can act as both reducing and stabilizing agents [22, 23]. Thus, the biomolecules that are present in plant extracts can reduce ferrous or ferric ions to zero-valent iron and effectively prevent agglomeration of ZVIN [23, 24].
From the inspiration of several, methods were utilized for the synthesis of ZVIN using green techniques for avoiding toxic and high-expensive materials for the prevention of environmental pollution. In this study, green, eco-friendly and low-cost synthesis of CR flower extract mediated and stabilized ZVIN was first time reported for the effective adsorptive removal of Cr(VI) from the contaminated water. The optimization factors for Cr(VI) removal from aqueous solution using CR flower extract stabilized ZVIN under different variables namely, pH of the aqueous solution, the concentration of ZVIN (g/L), the dosage of ZVIN, contact time, and initial concentration of Cr(VI) were also investigated.
Ferric nitrate nonahydrate (FeNO3. 9H2O, CAS Number 7782-61-8), potassium dichromate (K2Cr2O7, CAS Number 7778-50-9), lead nitrate (Pb (NO3)2, CAS Number 10099-74-8), and ethyl alcohol (CH3CH2OH, CAS Number 64-17-5) were purchased AR grade from Sigma Aldrich Chemicals, India. The fresh flowers of Catharanthus roseus were collected from Osmania University, Hyderabad, India.
2.2 Collection of Catharanthus roseus flower extract
Catharanthus roseus flowers are washed twice with double distilled water and then cut into small pieces and dried. The CR flower extract was prepared by mixing 20 mg weight of dried flower extract powder with 100 mL double distilled water in a 250-mL conical flask and boiled for 5 min at 100 °C. After cooling, the solution is filtered by Whatman’s no.1 filter paper and the filtration carried out thrice to get a clear extract solution.
2.3 Eco-friendly preparation of ZVIN
2.4 Characterization techniques
UV-visible absorption analysis was carried out using UV-3600 spectrophotometer (Shimadzu) in the range of 200–800 nm. Scanning electron microscope (SEM) imaging analysis of the samples was conducted using a Zeiss evo18 scanning electron microscope with an equipment of EDX. The crystallinity and phase were examined using X-ray diffraction (XRD) analysis with a computer-controlled X-ray diffractometer (Philips, X’pert pro diffractometer) and equipped with a stepping motor and graphite crystal monochromator in the range 2θ = 10–80°. The FT-IR spectra of the synthesized ZVIN were analyzed by Bruker spectrophotometer in the range of wavenumber 400–4000 cm−1.
2.5 Batch adsorption experiments for Cr(VI) removal
2.6 Adsorption isotherms
Langmuir and Freundlich’s models are applied to the equilibrium experimental data to predict the adsorption mechanism of Cr (VI) removal using CR flower extract stabilized ZVIN.
The calculated Langmuir and Freundlich adsorption parameters for removal of Cr(VI) using ZVIN
Xm (mg g−1)
Kf (mg g−1(L mg−1)1/n)
B (L mg−1)
3 Results and discussion
3.1 UV-vis absorption analysis
3.2 FT-IR analysis
3.3 XRD analysis
3.4 SEM and EDX analysis
3.5 Effect of factors on Cr(VI) removal
Previous reports have been shown that many parameters influence the Cr(VI) removal ability of ZVIN in triplicate times for the optimization of parameters. In this study, the major factors known to Affect the reactivity, including the initial concentration of Cr(VI), contact time, pH, and ZVIN dosage are investigated for the removal of Cr(VI) from the contaminated water by adsorption technique.
3.5.1 Effect of initial concentration of Cr(VI)
3.5.2 Effect of contact time
3.5.3 Effect of solution pH on Cr(VI) removal
The pH of the solution shows a vital role in the adsorptive removal process of heavy metals due to the surface charge and distribution of binding sites of adsorbent are greatly influenced by the pH of the solution. Therefore, adsorption ability of adsorbent and charge of the metal ion in the solution both are controlled in response to pH of the medium. In this study, the original pH of the Cr(VI) solution without adjustment is 4.3 and the influence of pH on the removal of Cr(VI) is investigated by considering the pH range from 4.3 to10.0. When the pH is less than 4.3, it was not taken into account due to acidic dissolution, wipe-out of ZVIN, and demolished organic capping agents on ZVIN at low pH . From the experimental data, it is observed that Cr (VI) removal efficiency of ZVIN decreases with the increase in pH of the solution. The reason may be explained at higher pH: (1) there is a competition between negatively charged hydroxyl ions and Cr(VI) which exists mainly in the form of oxyanions and (2) the oxide passivated film formed due to the decrease in iron corrosion production, which prevents further adsorption of Cr(VI) ions.
3.5.4 Effect of dosage of ZVIN on Cr(VI) removal
The amount of ZVIN varied (0.4, 0.8, 1.2, and 1.6 g/L) to determine the influence of dosage of ZVIN on Cr(VI) removal. In this study, it is noticed that the amount of ZVIN increases the rate of removal efficiency and is also increased due to the increase in the number of binding sites expected. However, Cr(VI) removal efficiency of ZVIN is not significantly less at the lower dosage but the adsorption process is much slower. At the higher dosage (1.6 g/L) of ZVIN, a rapid adsorption takes place and the equilibrium is attained within 10 min while at the lower dosage (0.4 g/L) the time required to attain equilibrium is 120 min.
3.5.5 Adsorption isotherms
Figure 7 shows the Freundlich adsorption isotherm of Cr(VI) removal by ZVIN. Freundlich constants Kf and n are determined using slope and intercept values of the straight line, respectively. However, from all these parameters, it was concluded that the Langmuir adsorption model was well fitted than the Freundlich adsorption model to the adsorption of Cr(VI) which confirms homogeneous and monolayer adsorption.
3.5.6 Adsorption kinetics
In this summary, we report for the first time, the eco-friendly, non-toxic, and low-cost ZVIN synthesized by renewable and naturally occurring CR flower extract as both a reducing and stabilizing agent. The synthesized ZVIN were completely characterized by UV-vis, FT-IR, XRD, and SEM-EDX analysis for investigation of their optical, structural, and morphological properties. The UV-vis and FT-IR analyses confirm that the polyphenols and amino acids present in the flower extract are responsible for the formation of ZVIN. The crystallinity and size of the synthesized ZVIN are confirmed by XRD and SEM analysis. The estimated average size of ZVIN is less than 100 nm. The obtained results demonstrated that the synthesized ZVIN are very efficient in the adsorptive removal of Cr(VI) from contaminated water by Langmuir and Freundlich adsorption isotherms. The Langmuir isotherm was a well fit to the adsorption of Cr(VI) on ZVIN than the Freundlich isotherm and the adsorption process follows pseudo-second-order kinetics. The results of the graphs showed that the interaction effects of pH, ZVIN concentration, and contact time had the highest degree of influence on Cr(VI) removal efficiency, respectively. The optimum operation conditions were obtained with pH of aqueous solutions of 4.3, ZVIN concentration 1.6 g/L, and initial concentration of Cr(VI) is 10 ppm. In the future, this work was suggested for the removal of other pollutants from the wastewater in industrial applications.
The authors are thankful to DST-FIST and Department of Chemistry, Osmania University, Hyderabad, India.
All authors (KS, DA, PYS) have contributed to the writing of the manuscript. All authors read and approved the final manuscript.
Compliance with ethical standards
The authors declare that they have no competing interests.
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