Removal of arsenate from aqueous solution using nanoscale iron particles

Iron (Fe) Nanoparticles topic is often rises in scientific literature and there are many researchers concern with it seriously. Studying scientist’s works and publications it is possible select their main notions and features. In this regard, iron nanoparticles or nanopowder are spherical or faceted high surface area metal nanostructure particles. Observing publication it is easily to find such interpretation, that  nanoscale iron particles are typically 20-40 nanometers (nm) with specific surface area (SSA) in the 30 – 50 m 2 /g range and also available in with an average particle size of 100 nm range with a specific surface area of approximately 7 m 2 /g. In a such way, nano iron particles are available as a nanofluid through the AE Nanofluid production group. Nanofluids, at the same time, are generally defined as suspended nanoparticles in solution either using surfactant or surface charge technology. Moreover, applications for Iron Nanocrystals include in environmental clean up of carbon tetrachloride in contaminated groundwater, magnetic data storage and resonance imaging (MRI) and in coatings, plastics, nanowire, nanofiber and textiles and in certain alloy and catalyst applications. Now become understandable, that further research is being done for their potential electrical, dielectric, magnetic, optical, imaging, catalytic, biomedical and bioscience properties.

Iron nanoparticles information is generally available in most volumes. So, “iron nanoparticle technology is receiving attention for its potential as a remedial method for a wide variety of common environmental contaminants” was said in “Removal of Arsenate from Aqueous Solution Using Nanoscale Iron Particles” by  Ching Yuan and Hsing-Lung Lien.

In fact, removal of As(V) using nanoscale iron particles was examined in batch reactors. Analyzing this process, it is understandable that nanoscale iron particles, utilizing zero-valent iron with a diameter less than 100 nm as reactive materials, have been demonstrated to effectively remediate a wide variety of common environmental contaminants. How we can characterize nanoscale iron particles? Here is the answer: we can characterize nanoscale iron particles and their corrosion products using SEM-EDX, XRD, BET surface area analyzer and Laser Zee Meter. In such a way, SEM-EDX results indicated adsorption of arsenic onto the iron surface. Stopped at  XRD analysis, we can note, that was found the formation of iron corrosion products including lepidocrocite, magnetite and/or maghemite at a reaction period of 7 d. In this case, measurements of zeta potential revealed that the nanoscale iron particles have a zero point of charge at pH 4.4. As we know, increasing adsorption amounts of arsenic with decreasing pH can therefore be attributed to the positive surface charge of the particles at lower pH. However, the maximum adsorption capacity of nanoscale iron particles determined by the Langmuir equation was about 38.2 mg/g. After studying this information we can say, that normalization of the adsorption capacity to specific surface areas provides insight into the importance of iron types and the contact time of reactions in influencing arsenic uptake. This detailed information was taken from the “Removal of Arsenate from Aqueous Solution Using Nanoscale Iron Particles” by Ching Yuan and Hsing-Lung Lien, Department of Civil and Environmental Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan.

Analyzing nanoscale zero-valent iron (NZVI), we can see that it was synthesized and tested for the removal of As(III), which is a highly toxic, mobile, and predominant arsenic species in anoxic groundwater. Before this we wrote about SEM-EDX, AFM, and XRD which we used to characterize particle size, surface morphology, and corrosion layers formed on pristine NZVI and As(III)-treated NZVI. Here we pay our attention on AFM results and this results showed that particle size ranged from 1 to 120 nm. Elaborating XRD and SEM results, became understandable, that NZVI gradually converted to magnetite/maghemite corrosion products mixed with lepidocrocite over 60 d.

In a such way, arsenic(III) adsorption kinetics were rapid and occurred on a scale of minutes following a pseudo-first-order rate expression with observed reaction rate constants (kobs) of 0.07−1.3 min-1 (at varied NZVI concentration). These values are about 1000× higher than kobs literature values for As(III) adsorption on micron size ZVI. Using special literature we know about adsorption capacity, and it means, that the maximum As(III) adsorption capacity in batch experiments calculated by Freundlich adsorption isotherm was 3.5 mg of As(III)/g of NZVI. In this respect, it is worth mentioning the fact that laser light scattering (electrophoretic mobility measurement) confirmed NZVI−As(III) inner-sphere surface complexation. Nevertheless, the effects of competing anions showed HCO3, H4SiO40, and H2PO42- are potential interferences in the As(III) adsorption reaction. Above stated results suggest that NZVI is a suitable candidate for both in-situ and ex-situ groundwater treatment due to its high reactivity.

Founding on studying publications we can also analyze the transformation process and say, that transformation of halogenated organic compounds (HOCs) by zero-valent iron represents one of the latest innovative technologies for environmental remediation. For instance, iron can be used to construct a reactive wall in the path of a contaminated groundwater plume to degrade HOCs. We know about nanoscale particles and that they are characterized by high surface area to volume ratios and high reactivities. So, BET specific surface area of the synthesized metal particles is 33.5 m2/g. In comparison, a commercially available Fe powder (<10 μm) has a specific surface area of just 0.9 m2/g. Batch studies demonstrated us that these nanoscale particles can quickly and completely dechlorinate several chlorinated aliphatic compounds and a mixture of PCBs at relatively low metal to solution ratio (2−5 g/100 mL). Nevertheless, surface-area-normalized rate constants (KSA) are calculated to be 10−100 times higher than those of commercially available iron particles. (Synthesizing Nanoscale Iron Particles for Rapid and Complete Dechlorination of TCE and PCBs,Chuan-Bao Wang and Wei-xian Zhang, Department of Civil and Environmental Engineering, Lehigh University, Bethlehem, Pennsylvania 18015).

Let’s stop with more details on the arsenic removal. In fact, batch and column experiments were conducted to investigate the effect of dissolved oxygen (DO) and pH on arsenic removal with zero-valent iron [Fe(0)]. In works, which we used as an informational source, we find the next words: “Arsenic removal was dramatically affected by the DO content and the pH of the solution. Under oxic conditions, arsenate [As(V)] removal by Fe(0) filings was faster than arsenite [As(III)]. Greater than 99.8% of the As(V) was removed whereas 82.6% of the As(III) was removed at pH 6 after 9 h of mixing. When the solution was purged with nitrogen gas to remove DO, less than 10% of the As(III) and As(V) was removed. High DO content and low solution pH also increased the rate of iron corrosion”. And we can make a conclusion that the removal of arsenic by Fe(0) was attributed to adsorption by iron hydroxides generated from the oxic corrosion of Fe(0).

After this analysis we know column results, which indicated that a filtration system consisting of an iron column and a sand filter could be used for treatment of arsenic in drinking water.

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Using our research on this topic we found, that iron oxide nanoparticles (IONPs) can play a significant role in the cycling of heavy metals.

We found out that arsenite [As(III)] is highly toxic, mobile, and predominant species in arsenic-contaminated groundwater. In a such way, IONPs have been synthesized and tested for the removal of As(III) from arsenic contaminated water. Moreover in our work, we synthesized IONPs, as a finely divided loose nanopowder, using a chemical method involving a dispersion of the metal cations (Fe(3 +)) through polymer molecules of polyvinyl alcohol (PVA) in an aqueous medium. At the same time, transmission electron microscopic images corroborate the result of IONPs of 45 nm average size and the rhombohedral shape. Selective experiments, conducted with an initial concentration of 25 ppm of As(III), have demonstrated the maximum As(III) adsorption capacity (96%) in 2. gL(- 1) IONPs in water at pH 4.5-7.5. At any rate, at room temperature, the adsorption isotherm studies have revealed a better correlation with the Langmuir isotherm than the Freundlich isotherm. Analyzing all process we should pay attention on the results. So, the results reveal that the removal of the As(III) species from water is associated with the As(III) adsorption onto the IONPs followed by a surface hydrolysis of the iron species (Use of iron-based technologies in contaminated land and groundwater remediation. Science of The Total Environment, Volume 400, Issues 1-3, 1 August 2008, Pages 42-51. Andrew B. Cundy, Laurence Hopkinson and Raymond L.D).

Thus, taking into account all above mentioned, it is possible to conclude that, that reactions involving iron play a major role in the environmental cycling of a wide range of important organic, inorganic and radioactive contaminants.

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