Abstract
For the past 15 years, nanoscale metallic iron (nZVI) has been investigated as a new tool for the treatment of heavy metal contaminated water. The removal mechanisms depend on the type of heavy metals and their thermodynamic properties. A metal whose redox potential is more negative or close to the reduction potential of Fe(0) is removed by the reduction process, while the others will be mediated by precipitation, complexation or other sorption processes. This review summarises our contemporary knowledge of nZVI aqueous chemistry, synthesis methods, mechanisms and actions (practical experiences) of heavy metal (Cd, Cu and Pb) removal and challenges of nZVI practical applications. Its inner core (iron(0)) has reducing ability towards pollutants, while the iron oxide (FeO) outer shell provides reaction sites for chemisorption and electrostatic interactions with heavy metals. Emerging studies highlighted that nZVI surfaces will have negatively charged species at higher pH and have good affinity for the removal of positively charged species such as heavy metals. Different sizes, shapes and properties of nZVI have been produced using various methods. Ferric salt reduction methods are the most common methods to produce stable and fine graded nZVI. Higher uptake of copper(ii), lead(ii) and cadmium(ii) has also been reported by various scholars. Practical pilot tests have been conducted to remove heavy metals, which gave highly satisfactory results. Challenges such as agglomeration, sedimentation, magnetic susceptibility, sorption to other fine materials in aqueous solution and toxicity of microbiomes have been reported. Emerging studies have highlighted the prospects of industrial level application of nano zero valent particles for the remediation of heavy metals and other pollutants from various industries.
Highlights
The increasing rate of industrial development has induced the release of heavy metals, such as lead, copper, cadmium and others, and has created an in-evitable environment
Lead metal ion removal experiment from wastewater has indicated that the pH of the solution where lead ion is found affects the state or species of nanoscale zero-valent iron (nZVI) adsorbent used for adsorption; at lower pH, the surface oxide layer is positively charged where it is dominated by protonated ions and attract only the positive ions, whereas at higher pH its surface becomes negatively charged and forms surface complexes with heavy metals such as Cd, Cu, Pb and others.[50]
Li et al reported that the average Cu(II) removal efficiency of nZVI at operating conditions of 10.20 g LÀ1 nZVI and an agitation time of 100 min was greater than 96%, and achieved an uptake capacity of 250 mg gÀ1.45 Li et al (2017)[67] reported a 99% copper removal efficiency from aqueous solution in the presence of other
Summary
The increasing rate of industrial development has induced the release of heavy metals, such as lead, copper, cadmium and others, and has created an in-evitable environment. It's less toxicity and comparative abundance in nature led nZVI to wide applications in the eld of environmental pollution abatement.[1,14,15] Nano-zero valent iron (nZVI) has Fe(0) core and Fe oxide outer shell, wherein the core has reducing ability and the outer shell acts as a reaction site for chemisorption and electrostatic interactions. Mechanisms such as reduction, absorption, precipitation and mineralization have played a role in the removal of heavy metals from aqueous phases using nZVI.[1,15] When dissolved in water nZVI exhibits ligand like properties. A laboratory scale and eld level practical tests of heavy metal removal capacity of nZVI were collected and reviewed
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