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Journal of Nanotechnology, Volume 2012, pp 1-18; doi:10.1155/2012/461468
Abstract: Successful mine soil reclamation facilitates ecosystem recovery, minimizes adverse environmental impacts, creates additional lands for agricultural or forestry uses, and enhances the carbon (C) sequestration. Nanoparticles with extremely high reactivity and deliverability can be applied as amendments to improve soil quality, mitigate soil contaminations, ensure safe land–application of the conventional amendment materials (e.g., manures and biosolids), and enhance soil erosion control. However, there is no report on using nanoenhanced materials for mine soil reclamation. Through reviewing the up-to-date research results on using environment-friendly nanoparticles for agricultural soil quality improvement and for contaminated soil remediation, this paper synthesizes that these nanomaterials with high potentials for mine soil reclamation include zeolites, zero-valent iron nanoparticles, iron oxide nanoparticles, phosphate-based nanoparticles, iron sulfide nanoparticles and C nanotubes. Transport of these particles in the environment and their possible ecotoxicological effects are also discussed. Additionally, this article proposes a practical and economical approach to applying nanotechnology for mine soil reclamation: adding small amounts of nanoparticles to the conventional soil amendment materials and then applying the mixtures for soil quality improvements. Hence the cost of using nanoparticles is reduced and the benefits of both nanoparticles and the conventional amendment materials are harnessed.1. IntroductionEver since the commencement of industrial-scale mining of coal and other minerals, drastic environmental impacts have been recorded arising from both the mined lands and from the wastes left behind at the surface [1, 2]. The local landscape and the soil quality are among the most severely disturbed environmental components by the mining processes through directly stripping the vegetation and soil layers (open-pit mining) and/or through depositing the ores and mining wastes on the soils [3]. Dramatic alterations of the geological environment of the coal/ores and the mining wastes significantly reduce the chemical stability of the minerals, resulting in the release of the environmental disruptive chemicals into the soils and creating the “mine soils.” Typical mine soils often refer to the antecedent or original soils which are affected and degraded by the acid drainage and mining wastes. Practically, this type of soils also include the exposed parent materials due to accelerated soil erosion and/or the top soil removal for open pit mining, and the deposition of mining solid wastes. Although the properties vary from location to location depending on the local geochemistry, a mine soil is usually acidic, heavy-metal laden, nutrient depleted, highly compacted, and not favorable to plant growth [4].The strategy for mine soil reclamation is to minimize the environmental impacts of mining by restoring the mine soils and the local ecosystems to the antecedent levels. An adequate reclamation of mine soils not only benefits the local environment but also can contribute to improving the global environment through carbon (C) storage in biomass and in the soils, and thus off-setting the increase of CO2 emissions from industrial activities. The depleted and drastically disturbed mine soils have a larger potential of C storage over agricultural soils due to the fact that intensively cultivated soils contain relatively high soil organic matter (SOM) and further increasing the C sink capacity is difficult. In contrast, mine soils usually contain low soil organic C (SOC) and thus possess high C storage potentials. Taking full advantages of the available C sink capacity by growing vegetation at the abandoned mining sites would increase the atmospheric CO2 adsorption and enhance the terrestrial C sequestration [5–7].Reclamation of mine soils for C sequestration requires high quality of remediation techniques and treatments. It is not enough just to protect against acid mine drainage (AMD) and heavy metals from contaminating the ground and surface waters. The mine soils must be reclaimed in situ so that the grasses, crops, or trees could grow sustainably with limited management. On a long run, a quick establishment of the vegetation and enhancing microbial activities in a mine soil can improve soil quality by accentuating phytoremediation of the contaminants, decreasing soil erosion, and enhancing concentrations of SOM and plant nutrients.1.1. Mine Soil Quality and Amendment MaterialsHarnessing an effective CO2 sink of a mine soil site requires establishment and maintenance of a healthy forest or other vegetation cover for a time scale of at least 25 years. Soils with high quality are indispensable to support the vegetation that can thrive and sustain itself. However, mine soils, especially the gob piles or mining rock wastes, usually have poor soil quality such as low SOM content, low fertility, micronutrient imbalance or toxicity, low N and P availability, soil compaction caused by the grading operations, shallow soil depth, low moisture holding capacity, high electrical conductivity, high heavy metal contents, and extreme pH, which all adversely affect vegetation establishments and SOC sequestration [5]. Therefore, soil amendments and proper management are needed in order to improve the soil physical, chemical, and biological properties at a disturbed site for establishing vegetation and making it an effective atmospheric CO2 sink. Several natural minerals and agricultural, industrial and municipal wastes have been tried for these purposes as soil amendments. For example, manures [8, 9], composts [10], biosolids [9, 11, 12], and paper mill sludge [9] have been successfully applied to increase the SOM content in the mine soils. Limestones, zeolites [13], and coal combustion byproducts [14–16] (e.g., fly ash, bottom ash, and flue gas desulfurization (FGD) gypsum) have also been intensively researched in reducing mine soil acidity and decreasing the heavy metal toxicity and uptake by plants. A range of various commercial N, P, and K fertilizers have been used to provide adequate nutrients for the vegetation establishment at mining sites as well [8, 12, 17]. The land applications of these conventional amendment materials are also encouraged by the increasing demands on disposal and reuse of these industrial by-products and community wastes at low cost. However, various levels of heavy metals (e.g., Hg, Cd, Cr, and Pb) and other toxic elements (B, As, Se, and Mo) often occur in coal combustion by-products [18]. Nuisance odors, the potential of pathogen transmission, and presence of toxic and persistent organic chemicals and metals in biosolids have for the most part limited the use of land applications [19]. A survey study [20] on 9 different biosolids produced by municipal wastewater treatment plants in 7 USA states indicated that some biosolids were highly enriched in organic wastewater contaminants (OWCs), suggesting the land application of the solids might become a potential nonpoint source of OWCs into the environment. The OWCs included pharmaceuticals, hormones, detergent metabolites, fragrances, plasticizers, and pesticides. Therefore, new types of effective and environmentally safe soil amendment materials are urgently needed for mine soil reclamation.1.2. Using Nanoenhanced Materials as Soil AmendmentsNanotechnology is an advanced modern approach. It provides new types of materials which offer the unique and important solutions to the limitations of other conventional materials and have numerous applications [21]. Nanomaterials and nanostructures have nanoscaled dimensions that range from 1 to 100 nm and often exhibit novel and significantly changed physical, chemical, and biological properties as a result of their structure, larger specific surface area, and quantum effects that occur at the nanoscale [21]. Applications of nanotechnology in water treatment and purification have witnessed significant developments in recent years [22–24]. However, little progress has been made regarding the application of nanoparticles to improve agricultural soil quality and to reclaim the drastically disturbed lands. Lal [25] proposed that applying nanotechnology in agricultural sector was one of the available options to increase the agricultural production, solve environmental problems, and feed the world’s growing population. Hence, it is imperative to review the state of the science of nanotechnology that has potentials in mine soil reclamation and mine soil quality improvement and to explore the feasibility of using nanoenhanced materials as replacements for the conventional amendment materials in agriculture. The specific nanotechnology interested in this paper encompass those able to increase soil pH and fertility, improve soil physical structures, reduce mobility, availability, and toxicity of heavy metals and other environmental contaminants and those able to stabilize the soil components and abate soil erosion at a mining site. Therefore, the overall objectives of this paper are to (a) review the available literature on various environmentally-friendly nanoenhanced materials which could be used as in situ soil amendments for mine soil reclamation; (b) briefly discuss the transport and mobility of those nanoparticles in the environment as well as their possible ecotoxicological effects (if any); (c) propose a practical approach to application of the nanomaterials in mine soil reclamation at low cost and in a more environmentally friendly fashion.2. Nanomaterials for Soil Reclamation and Environmental RemediationReclamation of mine soils involves removing soil contaminants and enhancing soil quality and fertility. Nanotechnology is a promising approach for these purposes. Two advantages of nanomaterials over the traditional amendments for soil reclamation include the higher reactivity due to smaller particle size and highe
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