Nanoremediation

Currently, groundwater remediation is the most common commercial application of nanoremediation technologies.[7][8] Using nanomaterials, especially zero-valent metals (ZVMs), for groundwater remediation is an emerging approach that is promising due to the availability and effectiveness of many nanomaterials for degrading or sequestering contaminants.[13]

Nanotechnology offers the potential to effectively treat contaminants in situ, avoiding excavation or the need to pump contaminated water out of the ground. The process begins with nanoparticles being injected into a contaminated aquifer via an injection well. The nanoparticles are then transported by groundwater flow to the source of contamination. Upon contact, nanoparticles can sequester contaminants (via adsorption or complexation), immobilizing them, or they can degrade the contaminants to less harmful compounds. Contaminant transformations are typically redox reactions. When the nanoparticle is the oxidant or reductant, it is considered reactive.[13]

The ability to inject nanoparticles to the subsurface and transport them to the contaminant source is imperative for successful treatment. Reactive nanoparticles can be injected into a well where they will then be transported down gradient to the contaminated area. Drilling and packing a well is quite expensive. Direct push wells cost less than drilled wells and are the most often used delivery tool for remediation with nanoiron. A nanoparticle slurry can be injected along the vertical range of the probe to provide treatment to specific aquifer regions.[13]

The use of various nanomaterials, including carbon nanotubes and TiO₂, shows promise for treatment of surface water, including for purification, disinfection, and desalination.[9] Target contaminants in surface waters include heavy metals, organic contaminants, and pathogens. In this context, nanoparticles may be used as sorbents, as reactive agents (photocatalysts or redox agents), or in membranes used for nanofiltration.

Nanoparticles may assist in detecting trace levels of contaminants in field settings, contributing to effective remediation. Instruments that can operate outside of a laboratory often are not sensitive enough to detect trace contaminants. Rapid, portable, and cost-effective measurement systems for trace contaminants in groundwater and other environmental media would thus enhance contaminant detection and cleanup. One potential method is to separate the analyte from the sample and concentrate them to a smaller volume, easing detection and measurement. When small quantities of solid sorbents are used to absorb the target for concentration, this method is referred to as solid-phase microextraction.[14]

With their high reactivity and large surface area, nanoparticles may be effective sorbents to help concentrate target contaminants for solid-phase microextraction, particularly in the form of self-assembled monolayers on mesoporous supports. The mesoporous silica structure, made through a surfactant templated sol-gel process, gives these self-assembled monolayers high surface area and a rigid open pore structure. This material may be an effective sorbent for many targets, including heavy metals such as mercury, lead, and cadmium, chromate and arsenate, and radionuclides such as ⁹⁹Tc, ¹³⁷CS, uranium, and the actinides.[14]

As of 2012, nano zero-valent iron (nZVI) was the nanoscale material most commonly used in bench and field remediation tests.[2] nZVI may be mixed or coated with another metal, such as palladium, silver, or copper, that acts as a catalyst in what is called a bimetallic nanoparticle.[3] nZVI may also be emulsified with a surfactant and an oil, creating a membrane that enhances the nanoparticle's ability to interact with hydrophobic liquids and protects it against reactions with materials dissolved in water.[1][2] Commercial nZVI particle sizes may sometimes exceed true “nano” dimensions (100 nm or less in diameter).[3]

nZVI appears to be useful for degrading organic contaminants, including chlorinated organic compounds such as polychlorinated biphenyls (PCBs) and trichloroethene (TCE), as well as immobilizing or removing metals.[3][9] nZVI and other nanoparticles that do not require light can be injected belowground into the contaminated zone for in situ groundwater remediation and, potentially, soil remediation.

nZVI nanoparticles can be prepared by using sodium borohydride as the key reductant. NaBH₄ (0.2 M) is added into FeCl₃•6H₂ (0.05 M) solution (~1:1 volume ratio). Ferric iron is reduced via the following reaction:

4Fe³⁺ + 3BH⁻
₄ + 9H₂O → 4Fe⁰ + 3H₂BO⁻
₃ + 12H⁺ + 6H₂

Palladized Fe particles are prepared by soaking the nanoscale iron particles with an ethanol solution of 1wt% of palladium acetate ([Pd(C₂H₃O₂)2]₃). This causes the reduction and deposition of Pd on the Fe surface:

Pd²⁺ + Fe ⁰ → Pd⁰ + Fe²⁺

Similar methods may be used to prepared Fe/Pt, Fe/Ag, Fe/Ni, Fe/Co, and Fe/Cu bimetallic particles. With the above methods, nanoparticles of diameter 50-70 nm may be produced. The average specific surface area of Pd/Fe particles is about 35 m²/g. Ferrous iron salt has also been successfully used as the precursor.[15]

Titanium dioxide (TiO₂) is also a leading candidate for nanoremediation and wastewater treatment, although as of 2010 it is reported to have not yet been expanded to full-scale commercialization.[10] When exposed to ultraviolet light, such as in sunlight, titanium dioxide produces hydroxyl radicals, which are highly reactive and can oxidize contaminants. Hydroxyl radicals are used for water treatment in methods generally termed advanced oxidation processes. Because light is required for this reaction, TiO₂ is not appropriate for underground in situ remediation, but it may be used for wastewater treatment or pump-and-treat groundwater remediation.

TiO₂ is inexpensive, chemically stable, and insoluble in water. TiO₂ has a wide band gap energy (3.2 eV) that requires the use of UV light, as opposed to visible light only, for photocatalytic activation. To enhance the efficiency of its photocatalysis, research has investigated modifications to TiO₂ or alternative photocatalysts that might use a greater portion of photons in the visible light spectrum.[9][17] Potential modifications include doping TiO₂ with metals, nitrogen, or carbon.