ABSTRACT
Nanofer Zero-Valent Iron Nanoparticles: Surface Morphology, Structure and Reactivity with Contaminants
Mahmoud M. Eglal
Nanoscale zero valent iron is emerging as a new option to treat contaminated groundwater. Despite its high potential for environmental application, there is only a limited knowledge about the fundamental properties of nZVI, particularly, its structure, surface composition and the change in these characteristics in an aqueous media, as nanoparticles interact with aqueous contaminants. Nanofer zero valent iron (nanofer ZVI) is a new and innovative nanomaterial capable of removing organic as well as inorganic contaminants in water. It displays a decrease in agglomeration, when it is coated with Tetraethyl orthosilicate (TEOS). TEOS imparts an increase in reactivity and stability to nanofer ZVI. The present study investigates the structure and surface chemistry of nanofer ZVI to understand the mechanism of reactivity towards organic and inorganic contaminants in water. The characteristics of nanofer ZVI were determined using scanning electron microscope/electron dispersive spectroscope (SEM/EDS), transmission electron microscope (TEM) and X-ray diffraction (XRD). The nanoparticle size varied from 20 to100 nm and its surface area was in the range of 25-30 m2 g-1. The thickness of the oxidizing layer had a range of 2 to 4 nm.
The adsorption and the oxidation behavior of nanofer ZVI used for the removal of Cu (II), Pb (II), Cd (II) ions and TCE from aqueous solutions was investigated. The optimal pHs for Pb (II), Cu (II), Cd (II) and TCE removal were found to be 4.5, 4.8, 5.0 and 6.5 respectively. Test data were used to form the Langmuir and the Freundlich model isotherms. The maximum loading capacity was estimated as 270, 170, 110, 130 mg per gram of nanofer ZVI for Cu (II), Pb (II), Cd (II) and TCE respectively. The adsorption of metal ions were compared with their hydrated ionic radii and their electronegativity. TCE oxidation followed the dechlorination pathway resulting in nonhazardous byproducts. Kinetic experiments indicated that the adsorption of heavy metals [Pb (II), Cd (II), and Cu (II)] and TCE was very rapid during the initial step of 50 minutes, which was followed by a much slower second step that was related to the solid state diffusion rate and the available surface area. Removal rates of 99.7% for Pb (II), 99.2% for Cd (II), 99.9% for Cu (II), and 99.9 % for TCE were achieved in less than 180 minutes.
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The Lagergren model (LM) and the single diffusion model (SDM) were used to understand the removal mechanism associated with nanofer ZVI. The time interval for particles to agglomerate and settle was between 4-6 hrs. SEM/EDS images showed that the particle size increased from 50 nm to 2 μm due to the particle agglomeration.
The competitive adsorption and displacement of mixed metals are complicated processes that are influenced by several factors. The occurrence of more than one possible adsorption mechanism, on nanofer ZVI contributes to the multi-faceted nature of these interactions. The binding of metal ions is thought to depend on the hydrated ionic radii and the electronegativity of metals. The mechanisms for binary and multi adsorption involved are influenced by time, pH and initial adsorbent concentration as well as the presence and properties of competing metal ions in the solution. In the isotherm and kinetic studies performed for binary and multi metal adsorption experiments, compared to Pb II and Cd II, Cu II achieved the higher adsorption capacity during the initial 5 min. However, after 120 min, all metals achieved removal efficiency in the range of 95 to 99%. Comparing the results of single and competitive adsorption kinetic tests for all the three metals during the initial 5 min, the presence of other metals slightly reduced the removal efficiency.
A part of the studies was devoted to the removal of mixed organic and inorganic contaminants. To this end, the removal of TCE by nanofer ZVI in the presences of Cu II at different environmental conditions was investigated. The kinetics of TCE degradation by nanofer ZVI was observed. At a dosage of 25 mg of nanofer ZVI, only 45% TCE was removed. However, when 0.01M Cu II and 0.15 TCE were present, 80 % degradation of TCE was achieved due to the presence of Cu II. SEM/EDS images indicated that Cu II is reduced to form Cu0 and Cu2O. These formations are considered to be responsible for enhancing TCE degradation. Direct TCE degradation in presence of Cu II involves hydrogenolysis and β-elimination, while indirect reduction involves atomic hydrogen and no direct electron transfer from the metal to reactants. Most of the iron present in nanofer ZVI could get dissolved causing the generation of localized positive charge regions and form metal chlorides to maintain electro neutrality in the system. Local accumulation of hydrochloric acid inside the pits regenerates new reactive surfaces to serve as sources of continuous electron generation. However, no significant effect of TCE was noticed for either increasing or decreasing Cu II sequestering on the surface of nanofer ZVI.