METAL NANOCOMPOSITES AND OTHER NOVEL METAL NANOPARTICLES IN REMEDIATION

Iron-based Nanocomposites

Even though nZVI is widely used for various environmental remediation, its application in wastewater or groundwater remediation is inadequate owing to particles aggregation, low stability, and iron leaching. Besides, due to the high activity of nZVI, its surfaces should be air and oxygen protected to prevent the formation of oxide layer (Garcia et al. 2018). Thus, to address these issues various researchers have functionalized nZVI with various materials such as biochar (BC), activated carbon (AC), sulfide, and organic acids to improve its performance. Recently, Wei et al. (2019) synthesized nZVI on oak sawdust-derived biochar (nZVI/ВС) for the removal of nitrobenzene (NB). In their study, they experimented the efficacy of BC, nZVI and nZVI/ВС to remove NB. For BC, 28% of NB was removed in 30 min and reached equilibrium, whereas the removal of NB by nZVI was approximately 37% in 360 min, which was slower than BC. As for nZVI/BC, 94% of NB removal was achieved in 360 min. These showed that nZVI and BC nanocomposites had synergistic effect and thus, enhanced the removal efficiency of NB.

Compounds containing sulfur such as pyrite (FeS2), greigite (Fe3S4) and iron sulfide (FeS) are known to have strong affinity for mercury (Hg), owing to its elevated capacity of mercury sorption and lasting steadiness in contaminated soil (Jeong et al. 2007). Nevertheless, FeS nanoparticles tend to agglomerate, making it unsuitable for in situ soil remediation. To address this concern, numerous methods such as polymer-stabilized wet-chemical synthesis, reverse micelle and poly(amidoamine) dendrimer-stabilization have been developed to synthesize nanosized FeS particles with precise distribution of morphology and size particle (Liu et al. 2015). Remarkably, Xiong et al. (2009) fabricated a stable nanosized FeS particles with carboxymethyl cellulose (CMC). Their results demonstrated that nanosized FeS/СМС particles were highly operative in removing mercury (Hg2+) from sediment. The results obtained for batch tests showed that 97% of Hg2+ concentration was reduced, when 26.5 is the molar ration of FeS/Hg. Then, the column sediment treatment tests with a 0.5 g/L of nanosized FeS particle suspension presented that the concentration of Hg2+ was reduced by 67%.

Iron nanoparticles are generally used in remediation of heavy metal contaminated soils. However, their ability to remove lead (Pb2+) ions from contaminated soil is not extensively studied. Recently, Peng et al. (2019) experimented the potential of numerous absorbents for Pb ion removal from contaminated soil. The absorbents studied included nZVI, nZVI supported by BC (nZVI BC). FeS nanoparticles, BC (FeS/ВС) supported FeS, ferrous oxide (Fe304) nanoparticles and Fe304 supported by BC (Fe304/BC). Their findings revealed that BC could advance the configuration of nanoparticles and increase the surface area for adsorption. The efficient removal of Pb2+ by these nanosized iron-based particles showed significant difference, where 45.80%, 54.68%, 2.70%, 5.13%, 47.47%, and 30.51% were obtained by nZVI, nZVI/BC, FeS, FeS/ВС, Fe304 and Fe304/BC, respectively.

Remarkably, Mahmoud et al. (2017) have reported the application of silayting agents as appropriate reagents for surface nZVI functionalization. Therefore, Mahmoud et al. (2019) studied the functionalization of nZVI via encapsulation to produce nZVI/NH2 and nZVI/ED, respectively. The metal ions removal including Co2+, Cu2+, Zn2+, Cd2+, Hg2+ and Pb2+ from various water samples were studied and assessed using the micro-column technique. The results obtained by nZVI/NH2 nanocomposite were tabulated in Table 2 and nZVI/ ED nanocomposite in Table 3. The data established that nZVI/NH2 possess high selectivity towards Hg2T and Pb2-. Besides, the ability of nanosized nZVI/NH2 and nZVI/ED composites in removing radioactive 60Co and 65Zn from artificial liquid wastewater was tested. nZVI/NH2 was able to extract 94.57% and 97.30%, and nZVI/ED 96.31% and 98.57% of 60Co and 65Zn, respectively. Overall, their studies showed that nZVI/NH2 and nZVI/ED were able to remove various metal ions and radioactive isotope incredibly.

Table 2 Metal ions removal efficiency by nZVI/NH2 nanocomposite from various waters (Mahmoud et al. 2019)

Co2*

Си2*

Zn2*

Cit2*

Hg2*

Pb2*

Tap water

92.21

93.32

90.10

96.77

93.10

100.0

Sea water

94.90

91.42

97.20

98.20

92.00

100.0

Wastewater

94.79

98.34

98.00

96.93

99.55

96.25

Table 3 Metal ions removal efficiency by nZVI/ED nanocomposite from various waters (Mahmoud et al. 2019)

Co2+

Cu2+

Zn2+

C(t2*

Pb2+

Tap water

93.88

96.38

97.88

96.16

94.11

100.0

Sea water

94.12

98.50

97.96

97.00

95.52

100.0

Wastewater

93.80

91.20

97.15

93.71

93.80

100.0

Nevertheless, persulfate activation method has been widely used for environmental remediation. Persulfate generates sulfate radical (S04—), which possesses high reactivity in several pollutant degradation (Hao et al. 2014). Thus, Su et al. (2019) synthesized a novel iron oxyhydroxides (a-FeOOH) functionalized on the surface of graphene oxide carbon nanotubes (GO-CNTs) matrix as an activator of persulfate for decolorization treatment of Orange II O(II). Approximately 99% of OH was decolorized by a-FeOOH/GO-CNTs in comparison with a-FeOOH of 44% and GO-CNTs of 18%.

Nickel-Cobalt Alloy Nanoparticles

Arsenite with triple valency (As(III)) is highly noxious and is tedious to eliminate from water, compared to arsenate with pentavalency (As(V)). Thus, As(III) to (V) conversion is considered as a feasible method to solve arsenic pollution problems. Guo et al. (2019) studied the electrocatalytic arsenite conversion into arsenate using nickel-cobalt (NiCo) nanosized alloy particles loaded onto carbon nanotubes (NiCoNPs/C). The oxidation potential of NiCoNPs/C was compared with bare carbon electrode in 0.1 M electrolyte of KOH, in the presence or absence of sodium arsenite (0.01 M of NaAs02). The results obtained showed that no oxidation occurred for the bare carbon electrode, signifying that As(III) was not converted to As(V). However, an oxidation peak at 1.1 V was observed when NiCoNPs/C electrode was tested in KOH electrolyte without NaAs02. The appearance of oxidation peak was contributed by the bivalent species (M2+) to trivalent species (M3+) (M = Ni, Co) oxidation in KOH. Then, NaAs02 was added and the potential increased to 1.15 V, which can be attributed to the As(III) to As(V) oxidation. M3+ was reduced to M2+ in the existence of As(III) and thus, oxidized to As(V). Their findings showed that NiCoNPs/C nanocomposites have great potential to be used in toxic As(III) into As(V) conversion, providing a promising method to tackle arsenic pollution issues.

Titanium Oxide-based Nanocomposites

As discussed in Section 3.3, titanium oxide (Ti02) was largely applied for water and air remediation. Herein, Truppi et al. (2019) examined the efficiency of Ti02/Au nanorods in degrading contaminants in water using UV-vis light. The efficacy of Ti02/Au nanorods as photocatalysts was tested with methylene blue (MB) and nalidixic acid (NA). Results collected showed that Ti02/Au nanorods had highest efficiency when they were synthesized under 450°C calcination temperature. Under UV light, Ti02/Au nanorods successfully catalyzed MB and NA, 2.5 and 3.2 times higher, respectively, when compared with commercially available Ti02 P25 Evonik. Furthermore, the reaction time of Ti02/Au nanorods was faster by 13 times in comparison with bare Ti02-based catalysts for MB and NA. Albay et al. (2016) also synthesized the photocatalystic Ti02/phthalocyanine- copper(II) nanosized composite for dye removal in aqueous solution. CuPc/Ti02 showed 100% removal of Cr(VI) after 150 mins of irradiation time, yet only 58% of Cr(VI) was removed by Ti02. Their results showed that CuPc/Ti02 nanocomposites is a feasible and promising catalyst that eliminates Cr(VI) ions up to 10 mg/L via visible light irradiation.

Carboxymethyl Cellulose and Polyacrylamide Nanocomposites

Godiya et al. (2019) prepared novel nanocomposites of CMC and PAM (polyacrylamide) for heavy metal remediation of wastewater. The CMC/PAM nanosized composites showed strong affinity towards ions of copper (Cu2+), Pb2+ and Cd2+. The capacity of adsorption in CMC/ PAM is superior with a value of 227.3 mg/g, 312.5 mg/g, 256.4 mg/g for Cu2+, Pb2' and Cd2+, respectively. Then, the adsorbed Cu2+ was condensed in situ for the formation of nanosized copper particles that are loaded with CMC/PAM (Cu-CMC/PAM) and the catalytic ability was tested with 4-nitrophenol. Their results showed that Cu-CMC/PAM effectively eliminated 4-nitrophenol and 4-aminophenol, thus verifying the dual functionalities of Cu-CMC/PAM as adsorbent and catalyst in remediation.

Fe-Mn Oxide-based Nanocomposites

Fe-Mn oxide is a binary metal oxide that is present largely in the environment and offers high adsorption capacity towards As and selenium (Se) oxyanions due to their elevated precise surface area, anion exchange and concurrent redox reactions (Liu et al. 2015). An and Zhao (2012) studied a novel nanosized binary Fe-Mn oxide particles stabilized with starch and CMC for the elimination of As(III) and As(V), to enhance the binary metal oxide deliverability for water and soil remediation. The extreme capacity of adsorption for As(III) and As(V) was reported to be 338 and 272 mg/g, respectively, at 5.5 pH. The results were superior when compared with other non-stabilized Fe-Mn nanocomposites reported by other researchers. McCann et al. (2018) also studied the sorption and oxidation of in situ arsenic using binary Fe-Mn oxide on arsenic contaminated soil. According to their initial results, it was deduced that As(III) was initially modified to As(V) by the Mn oxide via oxidation, then simultaneous adsorption of As(V) to iron oxide. Adsorption of As(III) and (V) on Fe-Mn oxides was rapid with removal rate of 93% within the first 2 hours and reached equilibrium at 97% after 24 hours. Maximum capacity of Fe-Mn oxide to adsorb As(III) and (V) was achieved with value of 70 and 32 mg/g, respectively. Similarly, Lin et al. (2019) synthesized Fe-Mn oxide supported on BC (FM/BC) in the study of arsenic removal. Various different concentration ratios of FeS04-7H20 and KMn04 solutions were used to synthesize FMBCs and they found that As(III) removal efficiency improved when FMBC dosage increased and weight ratios of BC:FeS04:KMn04 of 18:3:1 had the highest As(III) removal efficiency. Their results showed that this nanocomposite possessed great potential in remediation of arsenic contamination.

Graphene Oxide-based Nanocomposites

Koushik et al. (2016) studied the delialogenation of pesticides and organics using condensed nanosized silver-oxide of graphene (rGO/Ag) composite. The results indicated that graphene oxide silver nanocomposite was able to degrade pesticides such as chlorpyrifos (CP), endosulfan (ES), and dichlorodiphenyldichloroethylene (DDE) completely in 60 min. Sarno et al. (2017) also emphasized the elimination of persistent pesticide, chlordane, using rGO/Ag nanocomposite. It was found that rGO/Ag completely removed chlordane in 11 min. The fast degradation of chlordance can be attributed to the absorbing characteristic of graphene and outstanding catalytic performance of the Ag. Dong et al. (2018) also synthesized a nanosized functional composite hydrogel, which was made from rGO with Fe304 nanoparticles and polyacrylamide (PAM). The proposed Fe304/rGO/PAM hydrogel exhibits high photocatalytic reaction for organic pollutant degradation, great capacity for heavy metal adsorption and outstanding mechanical strength. The ability of Fe304/rGO/PAM hydrogel to degrade organic dye was tested with Rhodamine В (RhB). Their results showed that 90% of 20 mg/L of RhB was removed under irradiation of visible light within 60 min. Noteworthy is the fact that the degradation efficiency of

Fe304/rG0/PAM hydrogel remained at 90% even after 10 cycles. In addition, synchronous removal of RhB and heavy metal ions was experimented. The results obtained showed that Fe304/rG0/PAM hydrogel was able to remove 90% of RhB and the heavy metal removal efficiency was in the range of 34.8 to 66.3%.

 
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