Iron Oxides Nanoparticles

In the past decades, iron oxide nanoparticles have garnered interest in environmental remediation owing to their excellent adsorption ability to arsenic (As), which is a very important water contaminant (Dixit and Hering 2003). Various nanosized oxides of iron particles are tested for As-contaminated water. Remarkably, Liang and Zhao (2014) tested the ability of iron oxide nanosized particles for As(V) remediation. As(V) laden soil with arsenic loading of 31.45 mg/kg was treated with 0.1 g of Fe/L. Their results showed that small fraction inclusions of the iron oxide nauoparticles reduced the As concentration by approximately 98%. Rajput et al. (2017) investigated the lead and copper ion remediation from groundwater via superparamagnetic iron oxide nanoparticles. The groundwater was spiked with 5 mg/L of lead and copper ions separately. 0.2 g/L of iron oxide nanoparticles was added to lead ions spiked groundwater and 0.1 g/L to copper ions spiked groundwater. Their results showed that 74% of lead ions and 28% of copper ions were removed from the groundwater. Besides, their findings also showed that there is no effect on the remediation efficiency of nanoparticles by other groundwater ions.

Magnetic iron oxide nanoparticles display great competence in wastewater treatment due to its high precise surface area, as it can remove contaminants from wastewater rapidly. Additionally, magnetic iron oxide can be reused by magnetic separation after treatment. Nassar et al. (2015) studied the application of nanosized magnetic oxides of iron particles to remove the presence of synthetic dyes in wastewater released from textile industry. Their results demonstrated that magnetic nanosized oxides of iron particles effectively adsorbed dye. The equilibrium of adsorption was attained within 50 min and 125 min for the prototype dyes and real textile wastewater, respectively. Zhao et al. (2018) investigated the use of porous iron oxides (Fe203) microcubes in organic pollutants and ion removal of heavy metals from polluted samples. The adsorption capacity of porous Fe203 microcubes was tested against humic acid, methyl blue, chromium ions [Cr(VI)] and lead ions [Pb(II)]. The synthesized porous Fe203 microcubes have large precise surface area of approximately 155 m2/g and consists of huge amount of Fe203 nanoparticles. Results obtained showed that porous Fe203 microcubes exhibited great capacity of adsorbing Cr(VI), Pb(II), humic acid and methyl blue with value of 175.5 mg/g, 97.8 mg/g, 159.4 mg/g and 4 Pb(II) 25.9 mg/g, respectively. Thus, porous-Fe203 was emphasized to be useful as a potential nanocatalyst in organic contaminant and ions of heavy metal remediation from water.

Silica (SiOz) Nanoparticles

Mesoporous silica (Si02) materials have gained attention in environmental remediation owing to their enhanced surface area, large volume of pores, facile surface modification and adjustable pore size which aid in adsorption (Tsai et al. 2016). Due to their excellent potential as adsorbents, Si02 has been extensively studied for the contaminant gas phase remediation. Moreover, various mesoporous Si02 with altered surfaces were tested to improve its adsorption capacity (Guerra et al. 2018). Grafting of functional groups is a noteworthy strategy to design new adsorbents in removing contaminants of interest (Huang et al. 2003). For example, silica has been functionalized with carboxylic acid, thiol, amino groups for heavy metals remediation from wastewater. Amine groups are generally incorporated into silica for acid removal from natural gas such as carbon dioxide (C02) and hydrogen sulfide (H2S). In this regard, Huang et al. (2003) synthesized the 3-aminopropyl-functionalized ordered mesoporous silica (MCM-48) in H2S and C02 removal from natural gas. Their results showed that the high efficiency of H2S and C02 removal was credited to high surface amine group availability of 3-aminopropyl-functionalized MCM-48. 80% of C02 was removed within 30 min at room temperature. Similarly, 80% of H2S was removed in 35 min. Wang et al. (2015) studied the amino-functionalized magnetic silica mesoporous structure for the lead (Pb2+) ion remediation. Magnetic silica was functionalized with aminopropyltriethoxysilane and thus, large quantity of amino groups was grafted on the surface. The amino-functionalized magnetic mesoporous composite exhibited outstanding adsorption capacity for Pb2~ ions with a value of 243.9 mg/g. Recovery of the adsorbent could be done easily with an external magnetic field followed by acid treatment. Drese et al. (2011) and Choi et al. (2011) also emphasized the ability of amine-modified silica for the capture of C02. Their results showed that CO? absorbs reversibly to amine-modified silica and it remained stable after adsorption-desorption process of 50 cycles, making it a cost-effective absorbent. The absorption of C02 by amine-modified silica was fast and 90% of C02 was removed within the first few minutes of treatment. Thus, their results indicated that grafting of amine groups to silica exhibit greater performance and stability and is cheaper, making it a feasible substitute to traditional C02 capture (Qi et al. 2011).

Besides, materials with silica are broadly studied for dye remediation from contaminated wastewater. Moreover, functional groups such as carboxylic (-COOH) groups can form hydrogen bonds with various pollutants. In this regard, Tsai et al. (2016) examined the silica mesoporous nanostructures that are functionalized with -COOH groups for methylene blue remediation from water samples. Their results revealed that the methylene blue remediation was highly dependent on the pH of the sample. The maximum methylene blue uptake was achieved at pH 9 indicating that carboxylic acid functionalized silica can be an effective adsorbent at basic pH conditions. As for this reason, these materials might not be able to usable in all real textile wastewater treatment due to the limited pH range.

Titanium Oxide (ТЮ2) Nanoparticles

Titanium oxide (ТЮ2) is another frequently studied metal-based remediation material of water and air due to its nontoxicity, cost effectiveness and photocatalytic properties (Li et al. 2008). Ti02 nanoparticles can be easily activated by UV light making it an effective photocatalyst to remove contaminants in various media. Ti02 nanoparticles also produce oxidative agents with enhanced reactivity such as radicals of hydroxyl that destroy microorganisms such as fungi, bacteria, viruses, and algae (Li et al. 2014). Alizadeh Fard et al. (2013) investigated the photocatalytic petroleum aromatic hydrocarbon degradation in groundwater via nanofilms fabricated by Ti02 powder. For their experiment, 1.5 g/L of Ti02 nanoparticles was coated on glass beads. The initial concentration of benzene, toluene, ethylbenzene and xylene in the groundwater used was 508, 778, 934 and 631 pg/L, respectively. Their results showed that under natural light photocatalytic remediation, the total removal of benzene, toluene, ethylbenzene and xylene achieved was 61.2%, 77.4%, 86.7% and 58.9%, respectively. Similarly, Mammadov et al. (2017) also demonstrated the photocatalytic ethylene degradation in air by nanosized Ti02 particles coated on beads of glass. The maximum efficiency in degradation was reported to be 70%.

Due to high demand in agricultural products, the application of pesticides has been increasing worldwide. Soil and water contamination by these toxic non-biodegradable pesticides such as 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4-dichlorophenoxypropionic acid (2,4-DP) have caused serious environmental problems. Abdennouri et al. (2016) synthesized Ti02 and investigated their photocatalytic degradation ability of 2,4-D and 2,4-DP in water samples. Initial concentration of 20 mg/L of 2,4-D and 2,4-DP was used and after 90 mins of exposing to UV light, approximately 85% of 2,4-D and 75% of 2,4-DP were removed. This showed that these photocatalysts can efficiently degrade 2,4-D and 2,4-DP. Similarly, Abdennouri et al. (2015) also studied the photodegradation of herbicides using Ti02. Their results showed that almost half from the initial concentration of 2,4-D and 2,4-DP were degraded within 90 min of irradiation time.

Graphene Oxide

Pollutants in water samples such as heavy metals, pesticides and halogens needed to be specifically treated, due to their toxicity and prevalent occurrence in soil and water (Koushik et al. 2016). Graphene oxide (GO), owing to its plentiful oxygenous functional groups which include carboxylate, hydroxyl and epoxide, made it an excellent candidate in removing heavy metal ions and pesticides from wastewater (Liu et al. 2016). Therefore, numerous researchers are keen to study the application of GO in remediation field. Particularly, Zhao et al. (2011) synthesized adsorbents using GO for cobalt (Co(II)) and cadmium (Cd(II)) ion remediation from water. The elevated capacities of Cd(II) and Co(II) ion sorption over nanosheets of GO at pH 6 was 106.3 mg/g and at room temperature was 68.2 mg/g. Sun et al. (2012) also investigated the application of GO in the europium (Eu(III)) remediation from water samples. Eu(III) is commonly used in nuclear control applications because it is a good neutron absorber and is generally selected as a chemical trivalent actinides (An(III)) and lanthanides (Ln(III)) analogue. The results revealed that the capacity of Eu(III) adsorption on GO nanoparticles was highest at pH 6 with a value of 175.44 mg/g at room temperature. Besides, they also revealed that adsorption capacity of Eu(III) by GO is superior when compared with other adsorbents such as Ti02, aluminosilicate zeolite and multi-walled carbon nanotube. Thus, it is proved that GO nanoparticles can be utilized in the trivalent actinide and lanthanide remediation of pollution in environment. Additionally, Wang and Chen (2015) examined the adsorption of cadmium (Cd2~), 1-naphthol and naphthalene by graphene oxide. Their results showed that GO showed exclusive capacities of adsorption for a wide range of pH. Numerous functional carboxyl, hydroxyl and carbonyl groups are formed on the surface due to incomplete GO reduction. These functional groups displayed a strong attraction to Cd2+ when compared with naphthalene and 1-naphthol. In addition, Liu et al. (2016) studied the oxides of graphene with distinct degrees of oxidation property for Co(II) ion remediation. They synthesized oxidative GOs via treatment with ozone at times of bubbling such as 0, 2, 4 and 6 hours. Adsorption capacity of ozonized GOs towards Co(II) was tested with initial Co(II) concentration of 20mg/L at pH 6.8 and room temperature. Their results showed that 30%, 35%, 40% and 42% of Co(II) were removed by GO samples after 18 hours. It has been revealed that the fabricated GOs with regulated oxidation not only increased the absorption capacity, it also decreased the size of GO sheet which improved their abilities of dispersion in water samples. Therefore, it is deduced that GO are better adsorbents for heavy metal remediation from water. Maliyekkal et al. (2013) have experimented the absorbent property of reduced GO for pesticide removal from water. The results revealed that the reduced GO exhibited extraordinary potential in removing pesticides from water.

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