نانوکامپوزیت ها برای تصفیه آب: یک مقاله مروری Nano-Composites for Water Remediation: A Review

Water contamination is a major international problem caused by industrial, domestic and environmental infl uences. The United Nations estimates that 300–۵۰۰ million tons of heavy metals, solvents and other waste are released into the world’s water supplies each year as a harmful by-product of industrial activity. [ 1 ] Water contamination can also be naturally derived. For example, arsenic contamination is a serious issue in countries such as Bangladesh, West Bengal (India) and Nepal due to the weathering of rocks that naturally contain arsenic. [ 2–۶ ] Furthermore, as global populations continue to grow the human pressure exerted on our water supplies is expected to intensify with potentially greater likelihood of pollution. Over the past decade nano-technology has been increasingly investigated as a potential replacement for traditional treatment methods and reactive agents in order to deliver clean water at a reduced cost whilst simultaneously meeting increasingly stringent water quality standards. [ 7 ] However, the exact defi nitions of ‘nano-scale’ and ‘nano-material’ are still subjects of controversy. In 2010, the Joint Research Centre (JCR) of the European Commission published a report highlighting the international range of defi nitions. [ 8 ] Just within the UK two defi nitions were found for the term nano-scale; the UK Department for Environment, Food and Rural Affairs (DEFRA) defi ned it as 200 nm, whilst other organizations used 100 nm. Following recommendations made by the JCR, in October 2011 the European Commission adopted the following defi nition of ‘nano-material’ for regulatory purposes; [ 9 ] A natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions are in the size range 1 nm – ۱۰۰ nm. Due to their miniscule size, nano-materials exhibit different physical, chemical and biological characteristics when compared to their larger, micro- and macro-scale counterparts (<100 nm). [ 8,10–۱۳ ] The nano-materials have a larger surface area to volume ratio and consequently a higher density of surface reaction sites per unit mass. Furthermore, surface free-energy is observed to be greater for nano-materials than for micro- or macro-scale counterparts. Nano-materials, therefore, display a higher reactivity for surface mediated processes. However, as the particle size approaches the electron mean-free path and wavelength scales (below approximately 30 nm), quantum size effects become apparent and fundamental physical characteristics are signifi cantly changed again. These effects can counteract the increased reactivity as demonstrated by Sharma et al., [ 14 ] with many further comprehensive studies of properties specifi c to nano-materials readily found in literature. When within the optimum size range, nano-materials potentially represent a more effi cient alternative to current materials used for water treatment. [ 7 ] A rapidly emerging technology already achieving commercial use in America is nano-particle (NP) injection. [ 10,15–۱۸ ] The NPs, usually zero valent iron nanoparticles (INPs), are injected into the ground as a dry powder or slurry to directly treat contaminated groundwater. The NPs can be either deliberately immobilized, and hence perform as a deep underground permeable reactive barrier (PRB), or mobilized, allowing the NPs to migrate with the contaminated plume of water ( Figure 1 ). This technique, however, highlights multiple disadvantages of using ‘free’ NPs for remediation including the important fact that NP behavior is still not fully understood. It is well recognized that the dispersion of NPs through a groundwater system will be limited by multiple processes; mineral sorption, microbiological activity, aggregation and formation of voluminous corrosion products. [ 10,13,18–۲۴ ] INPs are particularly prone to aggregation and sedimentation because of their strong magnetic properties [ 21,25 ] in addition to electrostatic NP-NP attractions which operate most effectively in concentrated particle suspensions (i.e. slurries). Multiple studies in the literature have developed methods for avoiding these problems by adapting the NPs themselves, as illustrated in Figure 2 , to limit or negate inter-particle attractions. Surfactants [ 10,18,26–۳۲ ] or polymers [ 22,33–۶۲ ] can be added to the NP surfaces to encourage steric hindrance and alter the surface charge to prevent electrostatic attraction. The NPs can also be incorporated into other mobile structures such as carbon forms, [ 63–۷۸ ] silica [ 79–۸۷ ] and colloidal clays. [ 88–۹۳ ] However, the exact transport and retardation mechanisms occurring within the ground are unique to each treatment scenario and dictated by multiple factors that can vary over time, including soil composition, fl ow rates, pH and Eh balance and bacterial communities. These variables are diffi cult to predict and would require unique tailoring of the NPs for each situation. Changes in the groundwater system (natural or otherwise) may also cause contaminants adsorbed to the surface of NPs to become remobilized and surface adaptations to be reversed or become redundant – consequences that become inevitable when considering the operational diffi culty of removing the NPs from the ground. Furthermore, there is relatively little known about the long-term ecotoxicological effects of freely dispersed NPs in the environment [ 49,94–۱۰۱ ] – the same properties that provide the remediative qualities could also make them hazardous for living organisms. If, in the future, NPs themselves are proven as an unacceptable toxic risk then contractors that have deployed NPs via injection may subsequently become liable for NP clean-up. As remediation methods need to have non-toxic reaction agents providing long term and stable removal mechanisms, the disadvantages highlighted make it diffi cult to establish whether this technology, as it currently exists, can be safely applied. Hence, although there is currently no argument for or against toxicity, the UK is taking a precautionary approach for introducing engineered NPs into the environment. This action follows reports by the Royal Society and Royal Academy of Engineering (2004) [ 99 ] and CL:AIRE (Contaminated Land: Applications in Real Environments), for the UK Department for Environment, Food and Rural A airs (2011). [ 96 ] Both reports highlight the need for more fundamental research into nanotoxicology and NP behavior in subsurface environmental systems. To avoid the limitations outlined it would be highly advantageous to develop a remediation method that utilizes the reactivity of NPs whilst avoiding the release of free NPs into the environment. One possible route is to develop a ‘nano-composite’, a product defi ned as; [ 102 ] A multiphase material where at least one of the constituent phases has one dimension less than 100 nm. Recent research has spawned a multitude of different permutations of nano-composite, where generally the NPs are combined with a micro- or macro-scale support material. In this arrangement the nano-reactivity is still exhibited and complemented by the properties of the accompanying material. The current article provides a review of emerging iron and iron oxide containing nano-composites that can be used in static water treatment systems, including permeable reactive barriers, batch reactor systems and point-of-use fi lters. These systems should avoid the problems associated with uncontrolled NP dispersions by holding them, and sorbed contaminants, securely within a stable structure. Iron and iron oxide NPs [ 103 ] are of particular interest because bulk iron has been used in treatment methods for many years and, as a miniscule derivative, INPs have been thoroughly studied for remediation purposes (see Crane and Scott (2012), [ 10 ] Zhang [ 104 ] and all references therein), although primarily for synthetic laboratory solutions. Most signifi cantly, they have been shown to remediate an impressive range of contaminants, [ 105 ] from heavy metals via adsorption [ 2–۶,۱۰۶–۱۱۴ ] to the degradation of chlorinated solvents via chemical reduction, [ 115–۱۲۱ ] and at much greater rates than bulk iron. Although this type of technology looks promising, this review will also highlight areas for research and development that require further improvement if nano-composites are to be a viable realistic water clean-up technology. One major issue that becomes apparent within this article is that there is little to no consistency in performance testing for nano-composites developed by different groups. This makes it very diffi cult to compare products and decide which are the most promising for further funding and upscaling.