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Acid mine drainage (AMD) is a significant source of water pollution in areas surrounding current and historic metal and coal mine sites. The high concentrations of soluble ferrous and non-ferrous metal sulfates found in AMD can have detrimental effects on the ground- and surface waters damaging natural ecosystems and impacting the health of humans and wildlife through bioaccumulation. Remediation of impacted waters requires cost-effective, sustainable approaches to extract toxic heavy metals and lower the risk posed by AMD contaminated aquifers.
Using Nanomaterials to Remove Heavy Metals from Water
The use of nanomaterials to remediate heavy metal-bearing waters and sediments is a growing trend due to their high adsorption capacity and their ability to immobilize problem contaminants. A variety of these nanomaterials exist, including nanoscale zerovalent iron (nZVI), nano-apatite based-materials (nAP), carbon nanotubes (CNTs) and titania nanoparticles (TiO2 NPs).
Heavy metals released from industrial activities, including mining, are typically deposited in sediments however they are not necessarily fixed in the substrate and can be recycled back into the overlying water column in response to physical and chemical changes.
Metals and metalloids including aluminium (Al), arsenic (As), chromium (present as the highly toxic Cr[VI]), lead (Pb), cadmium (Cd), copper (Cu), zinc (Zn), uranium (U) and cobalt (Co) have all been effectively treated using a range of nanomaterials including nZVI, nAP and CNTs. Nanomaterials’ high specific surface areas provide more active sorption sites, making them effective remediation materials.
Applications of Nanoscale nZVI
Nanoscale nZVI has been shown to selectively remove Cu, Cd and Al from AMD from a legacy mine site on Anglesey (Wales, UK) despite the presence of several other metals (sodium, calcium, magnesium, potassium, manganese, and zinc) in the wastewater. In addition to removal of Cu, Cd and Al the Cu was transformed from an aqueous phase to zerovalent copper nanoparticles on the surface of the nZVI, representing a potential high-value by-product as a result of remediation.
Issues with nZVI applications include a weak surface charge resulting in the formation of microscale aggregates reducing mobility and durability. Despite this significant advantages of using nZVI include their ability to reduce highly toxic heavy metal ions such as Cr(VI) to the far less mobile and toxic Cr(III), and Pb(II) to Pb(0).
Applications of Nanoscale nZVI
Nanoscale iron oxides represent another form of nanomaterial with proven success in remediation and removal of heavy metals from solution. The mineral magnetite (Fe3O4) is formed both chemically, precipitating from magmas or being chemically reduced from Fe(III)-oxyhydroxides, and biologically by dissimilatory metal-reducing bacteria (DMRB).
One species of bacteria highly efficient at forming biogenic magnetite is Geobacter sulfurreducens. Pairing reduction of Fe(III) to oxidation of an organic substrate such as acetate (CH3COO-) G. sulfurreducens is able to form nanoscale magnetite particles with a surface enriched in Fe(II). This surface enrichment in reduced Fe allows effective reduction and removal of heavy metal ions, including Cr(VI) and Cu(II), from various forms of wastewaters representing an excellent option for remediation of heavy metal impacted waters.
The benefit of forming of nanoscale biogenic magnetite in this fashion is that it does not require high-temperatures, pressures or significant amounts of chemicals, nor does the material have to be milled to reduce its size. Nanoparticle size can be tailored depending on the concentration of biomass added to the amorphous Fe(III)-oxyhydroxide precursor. Initial development of the process was carried out by Professors Jon Lloyd, Richard Pattrick, and Gerrit van der Laan and Dr. Victoria Coker, all from the University of Manchester. Numerous successful studies have been performed using “bio-magnetite” to remove contaminants by researchers from the University of Manchester’s Geomicrobiology Group, headed by Professor Jon Lloyd.
Both in-situ and ex-situ remediation strategies exist, but ex-situ approaches necessitate disturbing the sediment structure and can cause secondary pollution or deterioration in the quality of the water column. In-situ methods are also not without their teething troubles; there is no guarantee of amendments reaching deeper contaminated zones in the sediment when particle size is too large to allow them to penetrate down the sediment column. Nanomaterials have the advantage of being able to move into nanoscale spaces in the subsurface, and their small size allows them to be dispersed more evenly throughout the water column representing a potential way of removing heavy metals from waste- and contaminated waters.
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