Zeolite-based technology can provide a cost effective solution for stormwater treatment for the removal of toxic heavy metals under increasing demand of safe water from alternative sources. This paper reviews the currently available knowledge relating to the effect of properties of zeolites such as pore size, surface area and Si:Al ratio and the physico-chemical conditions of the system such as pH, temperature, initial metal concentration and zeolite concentration on heavy metal removal performance. The primary aims are, to consolidate available knowledge and identify knowledge gaps. It was established that an in-depth understanding of operational issues such as, diffusion of metal ions into the zeolite pore structure, pore clogging, zeolite surface coverage by particulates in stormwater as well as the effect of pH on stormwater quality in the presence of zeolites is essential for developing a zeolite-based technology for the treatment of polluted stormwater. The optimum zeolite concentration to treat typical volumes of stormwater and initial heavy metal concentrations in stormwater should also be considered as operational issues in this regard. Additionally, leaching of aluminium and sodium ions from the zeolite structure to solution were identified as key issues requiring further research in the effort to develop cost effective solutions for the removal of heavy metals from stormwater.
Zeolite Heavy Metals Stormwater Leaching Stormwater Reuse1. Introduction
Access to safe drinking water has been declared as a basic human right by the United Nations [1]. Some 884 million people do not have access to safe water due to the pollution of conventional water sources [2]. Additionally, rapid population growth and increased urbanisation have resulted in an unprecedented demand for both potable and non-potable water. In recent years, intensive efforts have been focused on utilising largely unused stormwater as an alternative water source as it is generally available in large volumes and easily accessible [3-5]. However, stormwater can contain harmful pollutants such as heavy metals, hydrocarbons and pathogens when collected from various land surfaces [6,7]. Thus, a significant level of treatment is required before reuse. Numerous research studies have investigated the treatment of these pollutants [8-13].
Among stormwater pollutants, heavy metals are of particular interest due to their toxicity and persistence in the environment [14-16]. They can cause health problems in humans and animals ranging from irritation to cancers [17-19]. Several physico-chemical techniques, such as the use of detention ponds, coagulation-flocculation and chemical precipitation have been investigated for the removal of heavy metals [20-24]. However, most of these techniques have economical and environmental consequences such as generation of contaminated sludge and high capital and operational costs [25,26]. Consequently, research efforts have been directed towards adsorption using low cost sorbents such as peat moss, clays and zeolites as an alternative technique for the removal of heavy metals in stormwater due to their economical and environmental benefits.
Zeolites could provide a low cost solution, as these materials are generally less expensive compared to commercial sorbents such as activated carbon and can remove a number of harmful pollutants, including toxic heavy metals [27]. The performance of zeolites in the removal of heavy metals has been widely discussed in research literature and conclusions vary widely in relation to their treatment performance [28,29]. This can be primarily attributed to the differing experimental conditions and the properties of zeolite used in these studies. This emphasises the fact that an in-depth knowledge on the influence of physical and chemical factors on the performance of zeolite is essential for the development of approaches for the removal of heavy metals from polluted stormwater. In this context, a comprehensive stateof-the-art review of research literature was undertaken with the aim of consolidating currently available knowledge and to obtain a fundamental understanding regarding the influence of physical and chemical parameters on the heavy metal removal performance of zeolites. The objective of this review encompasses evaluating available zeolite-based approaches targeting removal of heavy metals from stormwater, currently available knowledge as well as to identifying any knowledge gaps that need to be addressed. Furthermore, with the view to highlighting the key issues in removing heavy metals from stormwater, the structure of zeolite and the influence of the properties of zeolite and experimental conditions are discussed.
2. Structure of Zeolite
A fundamental knowledge on the structure of zeolite is essential to better understand the effects of physico-chemical properties of zeolite and experimental conditions on heavy metal removal performance. Zeolite is an aluminosilicate porous material built on a three-dimensional network of SiO4 or AlO4 tetrahedral structures as illustrated in Figure 1 [30].
Various secondary structures such as cages and cavities are also found in zeolite due to different arrangement of the tetrahedral structures. The porous nature of zeolite is a result of these secondary structures.
The presence of trivalent Al atoms in the framework results in an anionic framework, which is neutralised by the extra framework cations such as sodium (Na+), calcium (Ca2+) and magnesium (Mg2+) [31]. Heavy metal cations in solution can be exchanged for these extra framework cations via ion exchange mechanism, which is, in certain cases, found to be the primary mechanism of heavy metal sorption employed by zeolite [32-34].
However, other mechanisms such as physisorption and chemisorption are also employed by zeolites for the sorption [35,36].
3. Sorption of Heavy Metals by Zeolite
Table 1 outlines the sorption capacities and selectivity series reported in research studies in order to highlight the different conclusions drawn by different studies. As evident in Table 1, the primary reason for the different selectivity series and sorption capacities is the use of different zeolite species. For example, in the study by Ouki and Kavannagh [32], two zeolite species, namely, clinoptilolite and chabazite were tested under the same experimental conditions and different selectivity series and sorption capacities were reported (Table 1). The main properties of zeolite that influence heavy metal removal performance are aluminium content or Si: Al ratio, pore size and surface area. On the other hand, the sorption capacities and selectivity series reported for clinoptilolite in different studies are different as shown in Table 1. Though, there can be some variations in the chemical composition of clinoptilolite due to their source of origin, the physical and chemical conditions used in each experiment were clearly different. Temperature, pH, zeolite concentration and initial metal concentration are the major parameters that are considered to have an effect on the removal of heavy metals.
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