Functional Green Carbon Nanocomposites for Heavy Metal
2.4 Advanced Removal Techniques in Water
2.4.5 Adsorption
The process of adsorption is widely used in the heavy metals treatment in aqueous solution. This is simply a mechanism through which the movement of mass takes place from the liquid to the surface of the solid by physical or chemical interactions. The key method of adsorption is based on the direct binding of cations/anions of metals to the surface of the adsorbent through
electrostatic attraction which leads to the elimination of more than 50% of heavy metals. Heavy metals are adsorbed to form flocs on the adsorbate sheet. The process is quite economic and widely utilized in water treatment systems. Most of the heavy metals removal using carbon-based green nano- composites takes place via adsorption process. In this context, polyacrylo- nitrile and ferric nitrate is used to produce fibrous and granular magnetic structural carbons (MCFs and MCPs) and showed major morphological dependence on the treatment of wastewater. An understanding of the ratio- nal designing of the adsorbent for removal purpose was understood. The experimental results advocated that the activity of fibrous nano-adsorbent increased from 12.6% to 51.4%. The hexavalent chromium removal was dependent on the initial Cr(VI) concentration which further depends on the increased content of Fe(NO3)3·9H2O in the precursor from 10% to 40%.
The enhanced removal of 4 mg/L is mainly due to the higher specific surface area of the fibril sample, resulting in a more active Cr(VI) adsorption site and the generated Cr(III) ions. Additionally, the stability of the fibril sample (MCFs-40) showed enhanced removal ability (43.17 mg/g), which was 3 times higher than the particulate nanosorbent (MCPs-40) which removed only 15.88 mg/g of hexavalent chromium. The removal mechanism of hexavalent chromium from the aqueous solution has been demonstrated in Figure 2.4. The fibrous samples can be found to be more suitable than other samples used for wastewater treatment. On treated MCPs-40 and MCFs-40, Cr(VI) and Cr(III) were identified, which suggested Cr(VI) and produced Cr(III) ions were adsorbed on the sample surface. The Cr(VI) removal
Before Cr(VI)
Cr(VI)
Removing After
Partly removed
Cr(VI)
10~20 min 10~20 min
Fe/Fe2+
MCPs MCFs
Fe2+/Fe3+ Cr(III)
Totally removed
Fe Fe2+/Fe3+
Fe2+
Cr6+
Cr6+
Cr3+
Cr3+
Fe3+
Fe2+/Fe3+
Fe3+
Fe2+ Cr6+
Cr6+
Cr3+
Cr3+
Fe
Figure 2.4 Removal mechanism of Cr(VI) over fibrillar and particulate magnetic carbon adsorbents (Obtained with permission from ACS) [62].
performance of MCFs-40 is nearly three times greater for the neutral solu- tion than that for MCPs-40, which is due to the more specific surface area of the MCFs-40. The cyclic stability of the designed adsorbent was also good till five consecutive experimental runs with Cr(VI) removal efficiency of 1.4 mg/g from MCFs-40 and 0.41 mg/g from MCPs-40 in neutral solution containing the initial Cr (VI) concentration at 4 mg/L [62].
One of recent study documented by utilizing activated carbon from spent coffee grounds and natural clay as novel green carbon nanocom- posite. The composite was functionalized by activation and magnetiza- tion process, simultaneously. The developed magnetic nanocomposite was employed for utilization and removal of Cu(II), Ni(II), and Pb(II) ions from aqueous solution. The optimal conditions for the treatment of heavy metals were; contact time-60 min, temperature- 25°C, pH ~5.5 and an ideal adsorbent dosage of 2 g/L. Mechanism of adsorption was under- stood by relaying the adsorption data to Langmuir, Freundlich, Temkin, and other kinetic models. Although, authors reported fair recyclability and regeneration of the adsorbent, but the biodegradability and feasibility studies was not reported. The research stated that the Langmuir adsorp- tion process is suitable and obtained higher adsorption capacity of 143.56, 96.16 and 84.86 mg/g for Pb(II), Cu(II) and Ni(II) respectively. The research also indicated that adsorbent material is effective for the removal of toxic heavy metals from wastewater [63].
In another strategy to the production of carbon-based green nano- composites, graphene oxide embedded calcium alginate (GOCA) beads were synthesized and further functionalized with poly(ethylenimine) to improve the adsorption potential towards heavy metal ions. The reported material has been used for removal for Pb(II), Hg(II), and Cd(II) from aqueous solution under different experimental conditions, where the func- tionalized beads were found to have high potential for adsorption rela- tive to the non-functionalized beads. The study also reports the value of uptake of metal ions to be 602, 374, and 181 mg/g for Pb(II), Hg(II), and Cd(II) ions, respectively. Moreover, through model fitting, it was under- stood that the removal of these heavy metals followed second order kinet- ics and monolayer adsorption took place as the adsorption data well fits the Langmuir adsorption theory. The as-synthesized green substance demonstrates a combination between adsorption and desorption up to five periods in sequence. This newly synthesized green adsorption beads demonstrated superior removal of metal ions in contrast to conventional materials and had a synergistic impact. The preparation mechanism, green materials functionalization, and efficient utilization as adsorbent for heavy metals removal are presented in Figure 2.5 [64].
The trend of utilizing polymer matrix and carbon nanocomposite simultaneously provides a unique opportunity to develop biodegradable green carbon polymeric nanocomposites. In this aspect, Zare et al. have made effort towards development of biodegradable polyaniline/dextrin conductive nanocomposites. Dextrin-based conducting nano-composites were synthesized in the presence of dextrin by polymerisation of aniline.
The analysis finds that the conductivity of green composites improves by
SA + GO solution Bead CaCl2 solution PEI functionalization
& PEI solution
Polyethylenimine Graphene oxide Alginate matrix Graphene oxide
=
=
1.
2. M+
PEI (a)
(b) SAGO: sodium alginate
: graphene oxide
Alginate matrix
in situ reduction preparation
Polyethylenimine Graphene oxide Metal ions Alginate matrix
O
O
O HN
HN
H2N H2N H2N
H2N
H2N HN
HN HN NH
HN
M
NH
NH O HN
O
O OH
OH OH
OH
OH OH
OH
COOH COOH
COO COOH
HC HC
N N
N N N
N N N
H2N N H2N
HN HN
NH NH NH
NH
N N N
N
N
N N
NH2
NH2 NH2
NH2 NH2
NH2 NH2
...2 NH2
NH2
NH2 NH2
NH2 a
a
Figure 2.5 (a) Beads preparation (i) SA and GO completely dispersed in water, (ii) SA-GO mixture made as beads in CaCl2 solution, and (iii) functionalization and reduction of GOCA beads in PEI at 40 °C; (b) The schematic representation and the possible chemical complexation of metal ion in the fGOCA beads. (Insert: Pictorial representation of fGOCA) [64].
rising the amount of polyaniline [65]. Furthermore, they report increase in the antioxidant capability with increment in the presence of aniline con- tent in nanocomposite. The nano-composite PANI/Dextrin showed a high activity of antioxidants of up to 70% which efficiently scavenged free radi- cals of 2, 2-Diphenyl-1-picrylhydrazyl (DPPH). The green material is used to remove heavy metal ions from the standard water solution including Cu(II), Cd(II) and Pb(II). Additionally, in vitro biodegradability of poly- aniline/dextrin nanocomposites with different weight ratios was studied by soil burial tests. The result established that the nanocomposites are bio- degradable under soil environment by degradation range between 30.18%
and 74.52%. This presents a good strategy and a significant observation in the direction of green carbon materials. The decomposition of the material under natural conditions is presented in Figure 2.6 [65]. However, all of the above reports with carbon-based green composites arises few significant inquiries, which is, the stability of the heavy metals in the environmental composite and its bioavailability.
III
15 30 45
Time (day)
Time (day)
60
15 30 45 60
IV
(a)
(b)
Figure 2.6 Decomposition of synthetic nanocomposites (a) III [Ani:Dex (1:3)] and (b) IV [Ani:Dex (2:1)] that were buried in the soil with natural microorganisms. (Ani-aniline and Dex-dextrin) [65].
An experimental attempt was made to synthesize magnetic nanocage (Mag@CNC) with pine resin and ferric nitrate salt as a source of carbon and iron. In order to prepare magnetic carbon nanocage (Mag@CNCs), it is treated at an elevated temperature under an inert atmosphere through carbonization. Microporosity and medium porosity demonstrate the higher surface area of green carbon nanocage through surface properties analysis. The material exhibits a uniform distribution of core-shell mag- netic nanoparticles in a carbon matrix, forming iron carbide (Fe3C) and metallic iron (α-Fe), having a size range of 20 to 100 nm, surrounded by a small amount of graphite layer wall. Mag@CNCs material was used to study adsorption of arsenic (III) species. The resultant analysis suggested that As(III) is combined at two types of surface positions of Mag@CNCs, namely, on the carbon surface material of the nanoparticles (≡CxOH2) and the Fe-oxide layer (≡FeOH2). This indicates that the advanced morphology and surface-driven synergistic properties of Mag @CNC materials are the basis for their As(III)-absorption properties [66]. New resources such as hazelnut shell, rice husk, pecan shells, jackfruit, maize cob or husk, fibers, and biomass contain higher carbon content which can be used as a green adsorbent for heavy metal uptake after chemical modification or conversion by heating into activated carbon/bio-carbon. Introduction of NM/metal oxide and ion exchange component not only enhances the binding capa- bility but also provides superior stability during treatment. These materials are considered as green material, since they are eco-friendly and does not affect the normal phenomena of ecosystem. Adsorption is popular and easy to use application with lower maintenance. Generally, there are three main processes involved in toxic sorption of green adsorbents: (i) the movement of the pollutant from the bulk solution to the surface of the sorbent; (ii) the adsorption of the surface of the sample; and (iii) transport within the sor- bent pores. Technological applicability and efficiency are essential factors in the selection of the most suitable adsorbent for the treatment of heavy metals in wastewater systems.
In the technology strategy of green carbon nanocomposites and wide applicability of the process “adsorption”, Kanmani et al. [67] have elab- orated applications of chitosan-based biopolymers and green materials for heavy metals treatment from wastewater systems. Similarly, there are other research materials reported which describes materials in terms as eco-friendliness but are yet to be exploited for heavy metals remediation purpose. The term of green composite is still unclear in some context which creates the need of its further studies with high novelty keeping in mind the sustainability of the environment.