Environmental Engineering Reference
In-Depth Information
16
OrganO-Clay nanOhybrid adsOrbents
in the remOval Of tOxiC metal iOns
Peng Liu
State Key Laboratory of Applied Organic Chemistry and Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization
of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, China
16.1
intrOduCtiOn
Toxic heavy metals in air, soil, and water lead to global ecological problems that are a growing threat to humanity [1].
Contamination of water by toxic heavy metals is a major environmental problem now. Heavy metals discharged in water bodies
through wastes also affect aquatic life and destroy their self-purification power [2]. In view of these facts, it is important to pre-
vent water pollution caused by heavy metals. Research is now focused on developing suitable clean and green technologies
either to prevent heavy metal pollution or to reduce it to very low levels. This can be achieved either by decreasing the afflux of
heavy metals to the receiving bodies (rivers, sewer and lake, etc.) or by their removal from contaminated media. The heavy
metal pollution arising from anthropogenic activities can be prevented, while that of natural origin is unavoidable.
Various treatment technologies have been developed for the removal of heavy metals from water and wastewater. The most
widely used conventional methods for removing heavy metals from wastewater include ion exchange, chemical precipitation,
reverse osmosis, evaporation, and membrane filtration. The type of treatment technique required for a particular industry
depends upon the nature, composition, and flow rate of the effluent together with the quality control needed to be achieved. The
effectiveness of the treatment plant can be optimized by adopting any one of the techniques discussed or the appropriate
combination of two or more techniques. But most of these methods suffer from some drawbacks, such as high capital and oper-
ational cost or the disposal of the residual metal sludge, and are not suitable for small-scale industries [3]. Besides high capital
and operational cost, most of the conventional methods are often ineffective or uneconomical when the heavy metal concentration
is in the range of 10-100 mg/l [4].
Adsorption has been found to be an efficient and economic process to remove heavy metals from industrial wastewater. The
process is suitable even when the metal ions are present in concentration as low as 1 mg/l [5]. Activated carbon (powdered or
granular) [6] and polymer ion exchangers and adsorbents [7] are the most widely used adsorbents in the removal of heavy
metals in water. However, the cost of the adsorbent becomes relatively high; therefore there is an increasing trend for substi-
tuting these sorbents with nonconventional adsorbents [8] (such as natural by-products [9], fly ash [10], sand [11], and so on)
in order to make the process economically feasible.
Clays are hydrous aluminosilicates broadly defined as those minerals that make up the colloid fraction (<2 µm) of soils, sed-
iments, rocks, and water [12] and may be composed of mixtures of fine-grained clay minerals and clay-sized crystals of other
minerals such as quartz, carbonate, and metal oxides. Clays play an important role in the environment by acting as a natural
scavenger of pollutants by taking up cations and anions either through ion exchange or through adsorption or both. Thus, clays
invariably contain exchangeable cations and anions held to the surface. Large specific surface area, chemical and mechanical
