Development of novel inorganic adsorbent for water treatment
Highlights
► Clay minerals and zeolites have cation exchange capacity and can be used to adsorb cationic pollutants. ► Cationic surfactant modified clays and zeolites are to be applicable for adsorption of anionic and organic containments. ► Layered double hydroxides (LDHs) possess good anion exchange capacity and are to be used to remove various oxyanions. ► Detailed studies are needed to explore the feasibility of replacing activated carbon with low-cost inorganic adsorbent. ► Future work on the LDHs is suggested to resolve the limitations of using this inorganic adsorbent in practical water treatment. ► Granulated format of inorganic adsorbents needs investigated further via the column tests in order to use them in practice.
Introduction
High quality water demand has increased owing to economic development and increasing in global population. On the contrary, increasing in water pollution level and implementing stringent drinking water standards should require water treatment plant to adapt efficient technologies to remove pollutants from water. Among all water treatment processes, adsorption is one of the most popular technologies and is considered as an effective, efficient, and economic method for water purification. The most common used adsorbent is activated carbon owing to its high surface area, porous structure, and special surface reactivity. Although it has great capacity of adsorbing various organic compounds, activated carbon has several disadvantages. It is expensive to use and the regeneration of exhausted activated carbon by chemical and thermal procedure will result in the loss of the sorbent.
Recently, a number of researches have been carried out to investigate and develop high efficient and cost effective adsorbents. Natural and synthesised/modified minerals are types of novel inorganic adsorbents. This article aims to review and provide new insight into recent development and assessing of natural/modified clay, zeolite and layered double hydroxides-based sorbents for the removal of aqueous contaminants.
Section snippets
Clay minerals and their modified forms
Clay is one of the most common earth's minerals, which are the residue of weathering or hypothermal action. The two major groups that clay minerals fall into are (1) kaolinites and (2) montmorillonites. When granite (igneous rock) is broken down, it first forms mica or illite that subsequently decomposes to kaolinites or montmorillonite.
Montmorillonite is an aluminium dioctahedral smectite clay mineral (Figure 1). It has a 2:1 configuration consisting of two silicon-oxygen tetrahedral sheets
Layered double hydroxides (LDHs)
In recent years, layered double hydroxides (LDHs) or hydrotalcite-like compounds have been studied substantially. The structure of LDHs can be described as a cadmium iodide-type layered hydroxide (e.g. brucite, Mg(OH)2), where a fraction of the divalent cations coordinated octahedrally by hydroxyl groups has been isomorphously replaced by trivalent cations. Some hydrogen-bonded water molecules may occupy the free space in the interlayer region [11]. The structure of LDHs and a typical
Natural/modified zeolites
Owing to their low cost and the capability of ion-exchange and adsorption, zeolites have attained a wide application to the environmental remediation in terms of separation, binding and chemical stabilisation of hazardous inorganic (e.g. NH4+, PO43− and heavy metals), organic (e.g. benzene, toluene, ethylbenzene and xylenes) and radioactive species (e.g. 137Cs, 90Sr) in soils and aqueous systems [25]. Zeolites are alumino-silicate minerals with a three dimensional porous structure based on
Discussions on the effect of specific surface area on the sorption performance
The specific surface area is one of the commonly measured properties in the evaluation of sorption performance of various adsorbents. Generally, the adsorbents with higher surface area are expected to have greater adsorption capacity than those with less surface area, owing to more exchange sites for the adsorbates. In case of LDHs, higher adsorption capacities have been observed for calcined LDHs (synthesised at 450–500 °C) compared to uncalcined LDHs (synthesised at 50–80 °C) owing to the
Concluding remarks and future work requirement
Both clay minerals and zeolites can be characterised by negatively charged framework structures, resulting from isomorphous replacement of Si4+ by Al3+. This negative charge imbalance is compensated by exchangeable cations (such as Na+, K+, Ca2+ and Mg2+) and thus they have CEC and can be used to adsorb various cations such as heavy metals. Although natural clay minerals and zeolites are not good adsorbents for adsorption of anionic ions and organics, surface modification using cationic
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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