CAs have attractive properties such as high porosity (80–98%), controllable pore structure, high surface area (400-900 m2/g), remarkable electrical conductivity (25–
100 S/cm), and thermal and mechanical properties [Tamon et al., 1997; Wu et al., 2006;
Li et al. 2007]. All of these properties make them promising materials for application in adsorption and as catalysts [Castilla and Hodar, 2005]. Moreover, CAs can also be used in capacitive deionization technology (CDT) water treatment (Figure 2.4), hydrogen generation and oxygen generation
Figure 2.4: Use of CDT in water treatment [Source: www.cdtwater.com, 2008]
CAs can also be used as electric double layer capacitors and materials for chromatographic separation [Tamon and Ishizaka, 1998; Gavalda et al., 2002], medical applications (controlled drug delivery system and drug targeting) [Tonanon et al., 2006], supercapacitors electrodes with large inner surface areas [Saliger et al., 1998], high electrical conductivity [Saliger et al., 1998] and high specific volume capacitance [Li et al., 2002].
Guilminot et al. (2007) prepared cellulose-based CAs and used them as a catalyst to support polymeric electrolyte membrane (PEM) fuel cell electrodes applications.
Aerogels have low thermal conductivity thus suitably used as thermal insulators, for example in refrigerators or heat storage devices [Gavalda et al., 2002]. Figure 2.5 below presents a comparison of the thermal insulation properties of aerogels with some commercially available insulating materials [Husing and Schubert, 2005].
Figure 2. 5: Comparison of thermal insulation properties amongst commercially available insulating materials (PUR≡ polyurethane foam; CFC ≡ chlorofluorocarbons;
EPS, XPS ≡ expanded and extruded polystyrene) [Husing and Schubert, 2005].
2.8.1 Carbon aerogel in HM removal
Currently, there have only been a few studies [Goel et al., 2005; Meena et al., 2005;
Kadirvelu et al., 2008] on the use of CAs in adsorption of HMs from wastewater. These studies investigated the performance of CAs in removing mercury, cadmium, lead,
copper, nickel, manganese, and zinc. The following paragraphs summarize the parameters which affected the HMs performance using CAs.
2.8.1.1 Parameters which affect the performance of CAs in HM removal
Parameters like pH levels, HM concentrations, temperature, and CA dosages have some effects on the adsorption performance.
pH is one of the important parameters controlling uptake of HMs from aqueous solutions.
The pH affects the hydrolysis of metal ions, resulting in the formation of metal hydroxide at high pH levels which precipitate and lead to low removal percentage [Yu, 1995; Meena et al., 2005; Rahman, 2007; Goel, 2006]. At lower pH levels, the metal is present predominantly as metal ions in the adsorptive solution; there is a competition between H+ and metal ions for adsorption at the ion-exchangeable sites, leading to a low removal of metal. The optimum pH level was found to be varying from one metal to another [Goel et al., 2005; Meena et al., 2005; Rahman, 2007]. The HMs concentration also affects the adsorption performance where the percent removal of HMs depends on the initial metal ion concentration and decreases with increase in initial metal ions concentration; this is because at high concentrations the numbers of HM ions are relatively higher compared to availability of adsorption sites [Meena et al., 2005; Rahman, 2007]. The effect of temperature on the adsorption of HMs was found to be proportional, as the temperature increased the adsorption increased. That is because with the temperature being increased the thickness of the boundary layer surrounding the adsorbent decreased, so that the mass transfer resistance of the adsorbate in the boundary layer decreases [Meena et al., 2005].
CA dosages have a paramount effect on the HM removal. The amount of CA affects the available exchangeable sites and the surface area which in other words affects the adsorption performance [Meena et al., 2005; Yu, 1995].
Meena et al. (2005) studied the removal of mercury, cadmium, lead, copper, nickel, manganese, and zinc by using CA as an adsorbent material. The study reported that the percentage of HMs removal depended on the initial metal ion concentration, which decreased with the increase in the initial HM ion concentration. It also depended on the adsorbent dose where the removal of HMs increased rapidly with the increase in the dose of adsorbents. For the same initial concentration of HMs, their percentage removal was
increased with the increase of contact time until equilibrium was attained. This study also found that at a constant initial metal concentration of 3 mg/L, adsorbent dose of 10 g/L and agitation period of 48h for all HM ions at varying pH on CA, the percentage adsorption increased with pH. It attained a maximum level at pH 6 for copper, nickel and zinc, whereas, for lead at pH 7 and for cadmium at pH 4. And thereafter the percentage decreased with further increases in pH.
Goel et al. (2005) used CA in removing mercury from aqueous solutions. The studies found that the amount of Hg (II) adsorbed by CA increased from 65% to 90% with the increase in temperature from 20 to 70 °C. The optimum condition for maximum removal was found at pH = 5.0 - 7.0 and adsorbent mass = 0.1 g and temperature = 70 °C. Goel et al. (2005) further studied the removal of three metal ions, mercury, lead, and nickel, onto CA. They investigated the metal ions removal using different parameters such as agitation time, metal ions' concentration, adsorbent dose and pH. The study found that increasing the initial solution pH level (2-10) and carbon concentration (50-500 mg per 50 ml) increased the removal of all three metal ions. The kinetic study showed that the pseudo second order kinetic model correlated well with the experimental data and better than the pseudo-first order model, where the thermodynamics study showed that the sorption was significantly increased with increasing the temperature.
Goel et al. (2006) studied the efficiency of removing cadmium using CA. The main findings were that the isotherm adsorption of Cd (II) followed the Langmuir model and the adsorption kinetics studies showed that the pseudo second order equation was better able to describe the adsorption of the cadmium ion than the first order equation of Lagergren .The studies also reported that the adsorption capacity from the Langmuir model was 15.53 mg/g for Cd (II) at an initial solution pH of 5.0.
The removal of lead, mercury and cadmium in mono- and multi-component (binary and tertiary) systems were investigated by Kadirvelu et al. (2008). The study found that the perfect adsorption of these metals in a single system was in the order of Hg (II) > Pb (II)
> Cd (II). However, in binary and tertiary systems the sorption was suppressed by the presence of other metal ions in aqueous solutions.