2.2 Heavy Metal Removal Methods
2.2.3 Adsorption
Adsorption is a well known method that is recognized as an effective and economical method for treating wastewater containing heavy metals owing to its simple process design and operation, high quality of the treated effluent and regeneration capacity of the adsorbents used. The following are some of the widely used adsorbents for heavy metal removal from wastewater.
2.2.3.1 Activated carbon
Large micropore and mesopore volumes and the resulting high surface area of activated carbon (AC) make this adsorbent extremely useful for the removal of heavy metals from wastewater. Additives of alginate (Park et al., 2007), tannic acid (Ucer et al., 2006), magnesium (Yanagisawa et al., 2010), surfactants (Ahn et al., 2009) and AC composite are
very effective for treating metallic wastewater. Converting carbonaceous materials into AC for heavy metal remediation has been reported in the literature. For instance, Kongsuwan et al. (2009) developed and used AC from eucalyptus bark for metal removal from binary component system. Guo et al. (2010) prepared AC from poultry litter and reported a very high adsorption affinity and capacity for heavy metals than commercial AC derived from bituminous coal and coconut shell.
2.2.3.2 Carbon nano-tubes
The discovery of carbon nano-tubes (CNTs) by Iijima (1991) has led to various studies due to its excellent properties and application towards removing heavy metals from wastewater (Kandah and Meunier, 2007; Wang et al., 2007, Kabbashi et al., 2009; Kuo and Lin, 2009;
Pillay et al., 2009; Li et al., 2010). It is reported that oxidation of CNTs by HNO3, NaClO and KMnO4 can significantly increase its sorption capacity.
Wang et al. (2007) in their study reported that acid treated CNTs showed better adsorption capacity for Pb over untreated CNTs. Pillay et al. (2009) investigated the adsorption capacity of functionalized multi-walled carbon nano-tubes (MWCNTs), non-functionalized MWCNTs and AC, and reported that both functionalized and non-functionalized MWCNTs showed a better adsorption capacity for Cr than that of AC. Despite the excellent properties of CNs for heavy metal removal from wastewater, extensive usage of CNTs for wastewater treatment may eventually result in the discharge of metal adsorbed CNTs to the environment which may further pose risk to the living things.
2.2.3.3 Low-cost adsorbents
Considering the high preparation cost of AC, the search for low-cost and easily available adsorbents for treating metal containing wastewater has been intensified. Till date, use of low-cost adsorbents, such as agricultural wastes, industrial by-products and natural products has been well reported for the treatment of wastewater containing heavy metals. Various industrial by-products, such as clino-pyrrhotite (Lu et al., 2006), lignite (Mohan and Chander, 2006), aragonite shells (Kohler et al., 2007), natural zeolites (Apiratikul and Pavasant, 2008a), kaolinite (Gu and Evans, 2008) and peat (Liu et al., 2008a), clay (Al-Jlil and Alsewailem, 2009), lignin (Betancur et al., 2009; Reyes et al., 2009), diatomite (Sheng et al., 2009), etc., have been successfully applied for treating metallic wastewater. However, industrial or large-scale use of these adsorbents is not well reported.
2.2.3.4 Biosorption
Biosorption has been proven to be a very promising method for the removal of heavy metals from aqueous solutions particularly at low concentration due to the use of highly effective and inexpensive bio-sorbents. Various forms of economical and non-living plant materials, such as black gram husk (Saeed et al., 2005), eggshell (Jai et al., 2007), potato peels (Aman et al., 2008), coffee husks (Oliveira et al., 2008), citrus peels (Schiewer and Patil, 2008), seed shells (Amudaa et al., 2009), sawdust (Kaczala et al., 2009), sugar-beet pectin gels (Mata et al., 2009), etc., have been broadly investigated for heavy metal removal. Due to its wide availability, low-cost and high metal sorption capacity, algae, a renewable natural biomass has drawn attention as a biosorbent for removing heavy metals (Apiratikul and Pavasant, 2008b).
Bio-removal of metals from wastewater has been investigated widely using several microbial species that include Bacillus cereus (Pan et al., 2007), Pseudomonas aeruginosa (Gabr et al., 2008; Tuzen et al., 2008), Escherichia coli (Quintelas et al., 2009; Souiri et al., 2009), etc. Fungi and yeasts are among the microbial species that are very easy to grow, yielding high amount of biomass and can be manipulated genetically and morphologically.
Various fungal species, such as Rhizopus arrhizus (Aksu and Balibek, 2007; Bahadir et al., 2007), Lentinus edodes (Bayramoglu and Arica, 2008), Saccharomyces cerevisiae (Chen and Wang, 2008; Cojocaru et al., 2009), Aspergillus niger (Amini et al., 2009; Tsekova et al., 2010), etc., are reported to be very efficient at heavy metal removal from wastewater.
Maximum metal removal capacities for Pb, Cu and Cd (70.7, 43.7 and 70.8 mg/g, respectively) were reported by Pakshirajan and Swaminathan (2006) in their study using a continuous packed column reactor immobilized with Phanerochaete chrysosporium.
However, the separation of bio-sorbents from metals would be difficult following the metal loading step (Fu and Wang, 2011).
Adsorption is a well recognized technique for handling wastewater containing low heavy metal concentration, but the application of AC is limited for treating wastewater on large- scale owing to its high cost. Even though several varieties of low-cost adsorbents have been developed and tested for treating heavy metal containing wastewater, metal removal efficiency depends on the type of adsorbent and also the wastewater characteristics.
Biosorption which is based on the use of biosorbents has been proven to be a very
promising method for the removal of heavy metal from wastewater since the last two decades (Fu and Wang, 2011).