Sage Journals: Discover world-class research

Abstract

The objective of the present study was to investigate the adsorption behavior of manganese oxide nanowires as an adsorbent for the removal of dissolved toluene from underground water. The nanowire membrane, composed of three-dimensional porous nanostructures plus superhydrophobic character, is very suitable for removal of the hydrophobic molecules. The effects of adsorbent dose, contact time, initial solution concentration, pH, salinity, and recyclability on the uptake of toluene by the adsorbent in batch mode are examined. The adsorption data compared to the adsorption of two kinds of activated carbons show that manganese oxide nanowire is competitive to activated carbons. Furthermore, the equilibrium data are fitted to different types of adsorption isotherms. Freundlich isotherm model illustrated the best fit to the data. The results of this study suggest that nanowires of manganese oxides can be used as a low cost, highly efficient adsorbent for the removal of dissolved hydrocarbon from aqueous solution.

Keywords

Dissolved toluene underground water nanowires manganese oxide adsorption isotherms

Introduction

Accidental or intentional leakage from oil reservoirs, gas stations, and pipelines are the most common cause of groundwater oil pollution. Consumption of contaminated groundwater can have serious environmental and health effects. Among the various petroleum hydrocarbons, benzene, toluene, ethylbenzene, xylene (known as BTEX) and their derivatives are the most common soil and groundwater contaminants (Picone, 2012). High water solubility of BTEX compounds relative to other petroleum components results in higher mobility in the subsurface water (Farhadian et al., 2008). Problems such as offensive odor, taste, and color appearance in underground water were clearly observed at a few kilometers far from the local refineries by our research group. The presence of such contaminants has a detrimental impact on the quality of life, drinking, and agricultural water in the area. Observations showed that the polluted water around local refineries contains compounds such as benzene, hexamethylbenzene, methylbenzene, toluene, ethylbenzene, etc. (Ansari and Habibagahi, 2006; Azimi and Habibagahi, 2003; Roghanian, 2008; Sadeghi, 2011; Talebi and Habibagahi, 2011). In this research, toluene was selected as a representative, because it has good solubility and less toxicity.

A short period of exposure time to the toluene at levels above the maximum contaminant level (for toluene concentration above 1 mg/l) may cause health effects such as kidney, liver, and nervous system problems. For the long-time exposure, more pronounced nervous impairments, disorders in speech, vision, memories, and liver and kidney damages are expected (EPA, 2009).

For soils and groundwater contaminated with oil, different treatment procedures have been proposed which are based on physical-chemical or biological processes. Among physical–chemical methods, air stripping, chemical oxidation, and adsorption are the most recommended methods (US Army, 1988). Techniques that incorporate methods such as flotation, gravity, and coalescing, which are traditionally employed to remove dispersed hydrocarbons, can only have a limited effect on the removal of dispersed hydrocarbons. Thus, other fundamental mechanisms like absorption, adsorption, extraction, membrane filtration, or oxidation have to be used in order to remove hydrocarbons (Meijer and Madin, 2010).

Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a molecular or atomic film (the adsorbate). Granular activated carbon (GAC) has been used successfully and recommended for the advanced (tertiary) treatment of municipal and industrial wastewater. However, it has some disadvantages (EPA, 2000), namely construction, maintenance and regeneration costs, and unsuitability for harsh conditions (e.g. temperature, pH, etc.) (Crittenden et al., 2012).

Many other kinds of adsorbing materials have been introduced. Sorbents for removing oil from water can be classified as inorganic mineral, synthetic organic, and natural organic materials. However, high cost, low sorption capacity, and environmental problems after usage are the main drawbacks of these techniques (Cojocaru et al., 2011; Korhonen et al., 2011; Payne et al., 2012; Radetic et al., 2008; Teas et al., 2001; Wang et al., 2012, 2010; Yuan and Chung, 2012; Zhu et al., 2011).

Meanwhile, several attempts have been made to invent high efficient materials (which means superporous and superhydrophobic); superabsorbent polymer materials (Kabiri et al., 2011; Kabiri and Zohuriaan-Mehr, 2003, 2004; Zohuriaan-Mehr and Kabiri, 2008), nanoporous materials like hydrogel composites (Nguyen et al., 2014), carbon nanotube (Hashim et al., 2012), nano manganese oxide nanowires (Yuan et al., 2008), and other materials (see Adebajo et al., 2003).

All of these attempts were made to structure a porous hydrophobic material. Hydrophobic and hydrophilic are frequently used descriptors of surfaces. A surface is hydrophobic if it tends not to adsorb water or be wetted by water and a surface is hydrophilic if it tends to adsorb water or be wetted by water. A simple, quantitative method for defining the relative degree of interaction of a liquid with a solid surface is the contact angle of a liquid droplet on a solid substrate. If the contact angle of water is less than 10°, the surface is designated superhydrophilic. For the contact angle less than 30°, more than 90°, and more than 150°, the surface is considered as hydrophilic, hydrophobic, and superhydrophobic, respectively (Arkles, 2011).

Nanowires of manganese oxides were first made and examined for decontamination of oily water by Yuan et al. (2008). Despite many synthesis methods to form one-dimensional manganese oxides (nanorods and nanowires) (Händel et al., 2013; Haris et al., 2014; Luo et al., 2000), their synthesis method conditions successfully fabricated elongated nanowires, forms an intertwined, self-assemble, and free-standing membrane. The final product was composed of superhydrophilic fibers that form a net of open superwetting capillaries coated with a superhydrophobic coating. They tested this membrane for a collection of organic solvents and dispersed oil, just to show the potential of the uptake capacity of the membrane. However, in their research, detailed study on the removal of dissolved hydrocarbons was not conducted to consider the effects of different parameters.

In this study, removal of dissolved toluene in water with nanowires of manganese oxide in different environmental conditions was investigated. Also, the synthesis process was optimized for the coating time. Effect of concentration, pH, salinity, and recyclability of this material was evaluated and the equilibrium data were fitted to different types of adsorption isotherms. For the sake of comparison, adsorption of toluene by two kinds of activated carbon (AC) was also evaluated. The results showed that this adsorbing material can be used in the purification systems such as permeable reactive barriers to decontaminate underground water from dissolved oil pollutions.

Materials and methods

Toluene (99%), manganese sulfate monohydrate MnSO₄·H₂O (MW 169.02), potassium sulfate K₂SO₄ (MW 174.27), and potassium persulfate K₂S₂O₈ (MW 270.35) were all purchased from Merck (Germany). Two kinds of GAC were obtained from Merck: charcoal activated catalog no. 1.02514.1000 and 1.02518.1000.

The surface morphology of the synthesized nanowires was analyzed using FE-SEM (HITACHI (S-4160)) for powder samples. X-ray diffractometry (XRD) spectra were obtained using a D8ADVANCED diffractometer operating at 2θ step size of 0.01° and speed 1°/min. Total organic carbon (TOC) was used to screen samples for toluene. TOC of samples was measured by Shimadzu 500 TOC analyzer. The Shimadzu TOC analyzer adopts the 680℃ combustion catalytic oxidation method. For each specimen, five samples were analyzed, by injecting 15 µl sample into the TOC analyzer. The results include total carbon (TC) and inorganic carbon (IC) and TOC (=TC−IC).

Synthesis

Nanowires were synthesized via a hydrothermal reaction described by Yuan et al. (2008). Reactants for this process consisted of 19.1 mmol of K₂SO₄, K₂S₂O₈, and MnSO₄·H₂O in a ratio of either 1:2:1, respectively, in 35 ml of deionized water. The solution was transferred to a Teflon vessel held in a stainless steel vessel. The sealed vessel was placed in an oven and heated at 250℃ for four days.

The product was suspended in 500 ml of deionized water and stirred for a day. The suspension was filtered and washed five times until all soluble impurities were removed from the solid. Moisture from the solid was removed by dispersing the solid on a Teflon substrate in an oven at 85℃ for 24 h.

The cryptomelane type of manganese oxide (K₂Mn₈O₁₆) was produced through the above-mentioned method. Hydrophilicity before coating and hydrophobicity after coating were evaluated as described later.

To transfer this material to a higher degree of hydrophilicity (which means a higher degree of hydrophobicity after coating), the following procedure was carried out as described by Yuan et al. (2008). The cryptomelane type of manganese oxide nanowires was first placed in a solution of NH₄Cl (27 g/l), resulting in ion exchange of K⁺ within the cryptomelane structure with NH₄⁺ cations. Heating of the nanowires which were doped with NH₄⁺ at 600℃ leads to the oxidation NH₄⁺ and subsequent conversion to N₂ gas. This product is bixbyite type of manganese oxide. For this type of manganese oxide, hydrophilicity before coating and hydrophobicity after coating were also measured. The morphology and crystallinity of the produced nanowires were analyzed via scanning electron microscopy (SEM) analysis and XRD.

Coating

The nanowire of manganese oxide is a superhydrophilic material. To use this material as an oil adsorbent, it needs to be coated with a hydrophobic material. Here, a simple vapor deposition method (VDM) was employed (Yuan and Dong, 2014; Yuan et al., 2008). A polydimethylsiloxane (PDMS) stamp and the nanowire membrane were placed in a sealed box heated at 250℃ for a specific time.

Batch tests

Solution preparation

The toluene solution was prepared by adding 0.52 g of toluene to 1000 ml of distilled water. Maximum solubility of toluene is 0.52 g/l (20℃) reported by Merck product information. The solution was stirred vigorously and then gently for 30 min at 25 ± 1℃. The solution was then poured in an aspirator bottle with stopcock and sat for 10 min to let the dispersed toluene float on the water surface. By the stopcock beneath the aspirator bottle, water–toluene solution was acquired.

Adsorbent preparation

A coated membrane made of nanowires of bixbyite type of manganese oxide was crushed, powdered, and sieved. The residual between sieve no. 10 to sieve no. 30 was taken and used in batch tests.

Tests

At the beginning of each batch test, the nanowires were placed into the autoclave glass bottles and the contaminant solution was then poured into the bottle and was immediately sealed. Adsorption tests were performed in 80 ml autoclave glass bottles, kept under continuous agitation (250 r/min) on a multiposition orbital stirrer. The temperature was maintained at 25 ± 1℃. Each batch test was conducted in duplicate to reduce the measurement errors.

Results and discussions

Synthesis

The morphology and crystallinity of the hydrothermally synthesized cryptomelane-type manganese oxide and bixbyite-type manganese oxide were analyzed via SEM analysis and XRD.

Figure 1 shows the X-ray powder diffraction pattern of the cryptomelane and bixbyite type of manganese oxide membranes. Almost all peaks can be assigned to diffraction from cryptomelane and bixbyite-type crystals (ICDD Cryptomelane-Q Reference code: 00-004-0778 and ICDD Bixbyite Reference code: 01-075-1573).

Figure 1.

X-ray powder diffraction pattern of the cryptomelane and bixbyite type of manganese oxide membranes.

A SEM image (Figure 2(a) and (b)) shows a view of the nanowire membrane. A large portion of the nanowires assemble in bundles over a length scale longer than several tens of micrometers, forming a net of open capillaries. From SEM results, the observed morphology of coated and noncoated nanowires was almost similar. However, it seems that coating the nanowires may change the distribution to a further porosity and the woven nanowires were unknitted. This result was observed over many SEM pictures gained from synthesized-coated nanowires.

Figure 2.

Characterization of the synthesized nanowire membrane. (a) SEM image of the noncoated nanowires, (b) SEM image of the silicone-coated nanowires, (c) FTIR spectrum of the silicone-coated manganese oxide nanowires, and (d) optical image of the cryptomelane membrane.

FTIR spectrum of the silicone-coated nanowires of manganese oxide is illustrated in Figure 2(c). The spectra interpretation was according to Tian et al. (2007) in which silicone-coated potassium bromide pellet with a similar procedure to the nanowire coating was conducted. The result shows the presence of short chains of PDMS on the nanowires.

In Table 1 some characterizations of coated and noncoated cryptomelane and bixbyite are listed. Water adsorption before coating was examined. The cryptomelane type of manganese oxide membrane could adsorb and maintain water up to 700% of its own dry weight. This increased for bixbyite type of manganese oxide up to 750% at the same conditions. This test was repeated several times and in each test the membrane was submerged in water and weighted before and after submergence (Figure 2(d)). After coating the membrane with PDMS via VDM, adsorption of water decreased to ∼20% of its own dry weight. The water contact angle of the silicone-coated cryptomelane and bixbyite membrane was measured. A camera captured pictures 1 min after drip on the membrane. The same silicone coating on the cryptomelane and bixbyite membrane and the difference between contact angles (127°–153°) confirm that the coating itself is not the sole cause of the superhydrophobic behavior and bixbyite-type manganese oxide has a further nanostructuring of the surface features (Yuan et al., 2008).

Table 1.

Characterizations of coated and noncoated cryptomelane and bixbyite.

	Contact angle (°)		Adsorbed water (%)
	Coated	Noncoated	Coated	Noncoated
Cryptomelane	127	∼10	30	700
Bixbyite	153	∼10	15	750

Effect of coating time and adsorbent doses

VDM was used to coat the nanowires to convert them from hydrophilic to hydrophobic material. As described, a PDMS stamp and nanowires were placed in a sealed container at 250℃. The optimum time in which the best coating was formed was obtained. Coating was carried out in 30, 60, and 120 min. Then, at different doses of adsorbents, the adsorption was measured. The results are illustrated in Figure 3. Obviously, increasing the adsorbent doses will increase the adsorption of toluene. However, increasing the coating time to 60 and 120 min does not show significant improvement in adsorption. So, the optimum time for coating was captured as 60 min for this type of coating for the rest of the experiments.

Figure 3.

Effect of coating time and adsorbent doses (time of stirring: 60 min).

Effect of contact time (Kinetic adsorption test)

In order to establish the equilibrium time of adsorption–desorption for maximum uptake of toluene from toluene–water solution, the amounts of toluene adsorbed on the adsorbents were studied as a function of stirring time, which varied from 10 to 360 min, using initial toluene concentration of 101.5 mg/l.

The relationship between the amount of toluene adsorbed as a function of the time is shown in Figure 4. It is clear that the amount of toluene adsorbed increased with increasing contact time. These data indicated that the reasonable time for adsorption equilibrium was 60 min. Therefore, 60 min was considered as an adequate time for the adsorption of toluene from toluene–water solution under the used operating conditions.

Figure 4.

Effect of contact time (kinetic adsorption test) (coating time: 60 min).

Effect of pH

The pH of underground water is affected by several factors. One of the most important factors is the soil composition and bedrock through which the water moves. Also, dumping the chemicals into the groundwater by industries is another factor which affects the pH. The effect of pH on toluene removal was carried out by adjusting pH of the toluene–water solution from 2 to 12 using 1 M HCl or 1 M NaOH prior to the addition of adsorbent. As it is shown in Figure 5, variations of pH from 2 to 7 (acidic solution) may have no effect on the adsorption. On the other hand, variations of pH from 7 to 12 will decrease the adsorption. This may have two explanations. One is that, alkaline environment may harm the coating. Subsequent tests showed no harmed coating. The combination of NaOH, H₂O, and toluene will make a polar molecule (IUPAC name: sodium; toluene; hydroxide; hydrate) and the superhydrophobic adsorbent is incapable of adsorbing the polar molecule (US National Library, Pubchem CID: 86738162). Therefore, the adsorption of toluene will decrease in NaOH alkaline environment.

Figure 5.

Effect of pH (time of stirring: 60 min, time of coating: 60 min).

Effect of saline water

Many underground water flows had been contaminated by saline water from the disposal of oil-field brine. The capacity of coated bixbyite type of manganese oxide to adsorb toluene from water–toluene solution at different salinities was investigated. Saline water was first prepared at different salinities (0, 100, 200, 250, 300 mg/l). Then, toluene was added to the solution. The results are shown in Figure 6. The adsorption decreased by increasing salinity. This is due to the reduction of solubility of toluene in saline water (Figure 6) (also see Keeley et al., 1988). Therefore, decreasing the solubility of toluene in water would decrease the amount of dissolved toluene in the container and adsorption rate of toluene will decrease accordingly due to lower initial toluene concentration in liquid phase.

Figure 6.

Effect of salinity on adsorption and solubility of toluene in water.

AC adsorption

The most recommended material to adsorb dissolved hydrocarbons is ACs. It has been studied, recommended, and used for many years (Adebajo et al., 2003; EPA, 2000; Okiel et al., 2011; Wu, 2004) but still has some disadvantages. Maintenance and regeneration of ACs are costly and have low stability in harsh conditions. Unlike ACs, nanowires of manganese oxide have good stability (Wang et al., 2012). Here, the adsorption of two kinds of ACs (Merck; Charcoal activated catalog no. 1.02514.1000 and 1.02518.1000 named: AC514 and AC518) is compared with manganese oxide nanowires. As it is shown in Figure 7, adsorption of manganese oxide nanowires is higher than that of ACs at the same adsorbent weight. The weight-base comparison is needed for economical evaluations.

Figure 7.

Comparison of adsorption of manganese oxide nanowire and activated carbons.

Effect of regeneration method

One of the most important economic issues when using adsorbent materials is their recyclability and regeneration. Two methods were used to regenerate the nanowires. In the first method, used nanowires were placed in an oven for 1 h at 100℃ and reused. In the second method, nanowires were placed at the room temperature at 25 ± 5℃ for one week and then reused. The results are depicted in Figure 8. From the results, it seems that heating the nanowires may harm the coating and reduce adsorption after each regeneration. Although, all the coating will be totally removed from nanowire surface at 360℃ (Yuan et al., 2008), heating up to 100℃ will impair the coating and the adsorption will be decreased. However, regeneration of the nanowires at the room temperature seems to save the workability of nanowires and the coating will be safe. The second method could be used just for hydrocarbons with high vapor pressure like BTE, which would evaporate at low (room) temperature. Also, a system could be designed and set to collect the evaporated hydrocarbons.

Figure 8.

Effect of regeneration method.

Adsorption isotherms

The adsorption isotherm studies were performed using samples of initial toluene concentrations of ∼90 mg/l, with adsorbent dosages of 0.01, 0.05, 0.08, 0.11 g/80 ml, and stirring to the equilibrium time which was determined previously. The results are given in Table 2. The data obtained in the series of batch tests were fitted to Freundlich and Langmuir isotherms.

Table 2.

Parameters of the Langmuir and Freundlich isotherm for the adsorption of toluene on to the coated bixbyite-type manganese oxide nanowires.

Langmuir isotherm parameters				Freundlich isotherm parameters
K_L	b	r	R ²	K_f	n	R ²
1.089	0.0024	0.84	0.12	1.41	1.11	0.86

To obtain the parameters of Langmuir isotherm (K_L and b), equilibrium experimental data for the nanowires of manganese oxide were analyzed by plotting C_e/q_e against C_e, in which the linearized form of the Langmuir equation is commonly written as

\frac{C_{e}}{q_{e}} = \frac{1}{K_{L}} + \frac{b}{K_{L}} C_{e}

(1)

where q_e is the amount of the substance adsorbed by the adsorbent (mg/g), C_e is the concentration of the substance remaining in equilibrium in the solution (mg/l), K_L is the amount of solute adsorbed/unit weight of an adsorbent in forming a complete monolayer on the surface (mg/g), and b is a fitting parameter related to the energy of adsorption.

Applying the Langmuir isotherm model (the graph of C_e/q_e versus C_e), it was observed that the linear trend line fitted to the adsorption data is not coinciding appropriately with the data and does not exhibit linear behavior and therefore the value of R² was not satisfactory (R²= 0.12). For the best fitted line, K_L is equal to 1.089, b (l/mg), and the Langmuir constant is equal to 0.0024.

On the other hand, the shape of the Langmuir adsorption isotherm is indicated by the separation factor or equilibrium parameter r. For r = 1, the adsorption is known as linear, 0<r<1, favorable, r>1, unfavorable, and for r = 0 irreversible. The r value calculated by equation (2) is 0.82 which falls between 0 and 1 confirming that the isotherm is favorable. The separation factor (r) is expressed in terms of a dimensionless constant as follows

r = \frac{1}{1 + {bC}_{0}}

(2)

where C₀ is the initial concentration of adsorbate (mg/l).

To obtain the parameters of Langmuir isotherm (K_f and n), the equilibrium experimental data for Freundlich isotherm were analyzed by plotting log q_e against log C_e, where the linear form of Freundlich isotherm model is expressed as

log q_{e} = log K_{f} + (\frac{1}{n}) \log C_{e}

(3)

where K_f is the Freundlich equilibrium constant which indicates the adsorptive capacity and n is the Freundlich constant indicative of the affinity of the adsorbate for the surface of adsorbent.

Applying the Freundlich model, the linear trend line fitted to the adsorption data had a better agreement than Langmuir isotherm with R²= 0.86. The K_f value is 1.41 and n value is 1.11. Therefore, based on the correlation coefficients, Freundlich isotherm was best to describe the sorption of toluene from toluene–water solution by the bixbyite type of manganese oxide nanowires. On the other hand, the high values of n (especially values between 1 and 10) indicate favorable adsorption (e.g. Errais et al., 2011; Girardello et al., 2016; Xiong and Mahmood, 2010). The n value found to be equal to 1.11 in our study indicating Freundlich adsorption is favorable.

The Langmuir and Freundlich isotherm relationships fitted to the experimental data are shown in Figure 9. Also, the isotherm parameters are listed in Table 2.

Figure 9.

Langmuir and Freundlich adsorption isotherm of toluene on nanowires of manganese oxide.

Conclusions

In this study, we investigated the possibility of using manganese oxide to separate the dissolved toluene from toluene–water solution. Hydrophilicity of two types of manganese oxide was evaluated. The results showed that noncoated (coated) bixbyite type of manganese oxide had higher hydrophilicity (hydrophobicity) than cryptomelane type. Several batch tests were conducted to evaluate the effects of adsorbent dose, contact time, initial solution concentration, pH, salinity, and recyclability on the uptake of toluene by the adsorbent. Bixbyite type of manganese oxide was found to be effective in the removal of toluene from aqueous solutions. Comparison between the results of adsorption of nanowires with two kinds of ACs indicated that the nanowires had higher adsorption at the same weight and nanowires are competitive to ACs. The isotherm analysis showed that Freundlich isotherm best describes the adsorption of toluene from toluene–water solution by nanowires of manganese oxide.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

References

Adebajo

Frost

Kloprogge

et al. (2003) Porous materials for oil spill cleanup: A review of synthesis and absorbing properties. Journal of Porous materials 10(3): 159–170.

Ansari Y and Habibagahi G (2006) Modeling of LNAPL spreading and migration to locate the source and fate of oil leaks. In: Seventh Int. Congress on civil engineering, Tehran, Iran, 7–9 May 2004.

Arkles

(2011) Hydrophobicity, Hydrophilicity and Silane Surface Modification, Morrisville: Gelest Inc.

Azimi K and Habibagahi G (2003) An optimization approach for groundwater monitoring network augmentation at shiraz oil refinery. In: Fourth Iranian hydraulic conference, Shiraz, 21–23 October 2003, pp.1–10.

Cojocaru

Macoveanu

Cretescu

(2011) Peat-based sorbents for the removal of oil spills from water surface: Application of artificial neural network modeling. Colloids and Surfaces A: Physicochemical and Engineering Aspects 384(1): 675–684.

Crittenden JC, Trussell RR, Hand DW, et al. (2012) MWH’s Water Treatment: Principles and Design. New York: John Wiley & Sons.

EPA (2000) Wastewater Technology Fact Sheet: Granular Activated Carbon Adsorption and Regeneration. EPA 832-F-00-017. Washington, DC: U.S. EPA.

EPA (2009) National Primary Drinking Water Regulations. EPA 816-F-09-004. Washington, DC: U.S. EPA.

Errais

Duplay

Darragi

et al. (2011) Efficient anionic dye adsorption on natural untreated clay: Kinetic study and thermodynamic parameters. Desalination 275(1): 74–81.

10.

Farhadian

Vachelard

Duchez

et al. (2008) In situ bioremediation of monoaromatic pollutants in groundwater: A review. Bioresource Technology Journal 99: 5296–5308.

11.

Girardello

Rovani

Giovanela

et al. (2016) Removal of pyrene from aqueous solutions by adsorption onto Brazilian peat samples. Adsorption Science and Technology 34(9–10): 538–551.

12.

Händel

Rennert

Totsche

(2013) Synthesis of cryptomelane-and birnessite-type manganese oxides at ambient pressure and temperature. Journal of Colloid and Interface Science 405: 44–50.

13.

Haris

MFL

Yin

Jiang

et al. (2014) Crystallinity and morphological evolution of hydrothermally synthesized potassium manganese oxide nanowires. Ceramics International 40(1): 1245–1250.

14.

Hashim

Narayanan

Romo-Herrera

et al. (2012) Covalently bonded three-dimensional carbon nanotube solids via boron induced nanojunctions. Scientific Reports 2: 363.

15.

Kabiri

Omidian

Zohuriaan-Mehr

et al. (2011) Superabsorbent hydrogel composites and nanocomposites: A review. Polymer Composites 32(2): 277–289.

16.

Kabiri

Zohuriaan-Mehr

(2003) Superabsorbent hydrogel composites. Polymers for Advanced Technologies 14(6): 438–444.

17.

Kabiri

Zohuriaan-Mehr

(2004) Porous superabsorbent hydrogel composites: Synthesis, morphology and swelling rate. Macromolecular Materials and Engineering 289(7): 653–661.

18.

Keeley

Hoffpauir

Meriwether

(1988) Solubility of aromatic hydrocarbons in water and sodium chloride solutions of different ionic strengths: Benzene and toluene. Chemical and Engineering Data 33(2): 87–89.

19.

Korhonen

Kettunen

Ras

et al. (2011) Hydrophobic nanocellulose aerogels as floating, sustainable, reusable, and recyclable oil absorbents. ACS Applied Materials and Interfaces 3(6): 1813–1816.

20.

Luo

Zhang

Huang

et al. (2000) Total oxidation of volatile organic compounds with hydrophobic cryptomelane-type octahedral molecular sieves. Microporous and Mesoporous Materials Journal 35: 209–217.

21.

Meijer DT and Madin C (2010) Removal of dissolved and dispersed hydrocarbons from oil and gas produced water with MPPE technology to reduce toxicity and allow water reuse. APPEA Journal 2010(1): 1–11.

22.

Nguyen

Feng

et al. (2014) Advanced thermal insulation and absorption properties of recycled cellulose aerogels. Colloids and Surfaces A: Physicochemical and Engineering Aspects 445: 128–134.

23.

Okiel

El-Sayed

El-Kady

(2011) Treatment of oil–water emulsions by adsorption onto activated carbon, bentonite and deposited carbon. Egyptian Journal of Petroleum 20(2): 9–15.

24.

Payne

Jackson

Aizpurua

et al. (2012) Oil spills abatement: Factors affecting oil uptake by cellulosic fibers. Environmental Science and Technology 46(14): 7725–7730.

25.

Picone S (2012) Transport and biodegradation of volatile organic compounds: Influence on vapor intrusion into buildings. PhD Thesis, Wageningen University, The Netherlands.

26.

Radetic

Ilic

Radojevic

et al. (2008) Efficiency of recycled wool-based nonwoven material for the removal of oils from water. Chemosphere 70(3): 525–530.

27.

Roghanian N (2008) An optimal pump and treat system for remediation of polluted groundwater under shiraz oil refinery using genetic algorithm. MSc Thesis, Department of Civil and Environmental Engineering, Shiraz University, Shiraz.

28.

Sadeghi L (2011) An investigation on the effect of temperature variation on LNAPL apparent thickness in the vadose zone. MSc Thesis, Department of Civil and Environmental Engineering, Shiraz University, Shiraz.

29.

Talebi A and Habibagahi G (2011) Evaluation of sharp interface approach in modeling of LNAPL spreading and migration on the water table. In: Proceeding of 14th Pan-American conference on soil mechanics and geotechnical engineering, Toronto, Ontario, Canada, October 2011.

30.

Teas

Kalligeros

Zanikos

et al. (2001) Investigation of the effectiveness of absorbent materials in oil spills clean up. Desalination 140(3): 259–264.

31.

Tian

Chen

Song

et al. (2007) Formation of alkali resistant PDMS–TiO₂–SiO₂ hybrid coatings. Materials Letters 61(22): 4432–4434.

32.

US Army (1988) Industrial and Oily Waste Water Control. MIL-HDBK-1005/9. Washington, DC: Department of Defense.

33.

U.S. National Center for Biotechnology Information. PubChem Compound Database; CID=86738162. Available at: https://pubchem.ncbi.nlm.nih.gov/compound/86738162 (accessed 24 January 2017).

34.

Wang

McLaughlin

Pfeffer

et al. (2012) Adsorption of oils from pure liquid and oil–water emulsion on hydrophobic silica aerogels. Separation and Purification Technology 99: 28–35.

35.

Wang

Silbaugh

Pfeffer

et al. (2010) Removal of emulsified oil from water by inverse fluidization of hydrophobic aerogels. Powder Technology 203(2): 298–309.

36.

Wang

Zheng

Wang

(2012) Superhydrophobic kapok fiber oil-absorbent: Preparation and high oil absorbency. Chemical Engineering Journal 213: 1–7.

37.

Wu J (2004) Modeling adsorption of organic compounds on activated carbon: A multivariate approach. PhD Thesis. University of Neuchatel, Neuchatel, Switzerland.

38.

Xiong

Mahmood

(2010) Adsorptive removal of phosphate from aqueous media by peat. Desalination 259(1): 59–64.

39.

Yuan

Chung

(2012) Novel solution to oil spill recovery: Using thermos degradable polyolefin oil superabsorbent polymer (oil–SAP). Energy and Fuels 26(8): 4896–4902.

40.

Yuan J and Dong H (2014) U.S. Patent No. 8,673,393. Washington, DC: U.S. Patent and Trademark Office.

41.

Yuan

Liu

Akbulut

et al. (2008) Superwetting nanowire membranes for selective absorption. Nature Nanotechnology 3(6): 332–336.

42.

Zhu

Pan

Liu

(2011) Facile removal and collection of oils from water surfaces through superhydrophobic and superoleophilic sponges. The Journal of Physical Chemistry C 115(35): 17464–17470.

43.

Zohuriaan-Mehr

Kabiri

(2008) Superabsorbent polymer materials: A review. Iranian Polymer Journal 17(6): 451.

Removal of dissolved toluene in underground water with nanowires of manganese oxide

Abstract

Keywords

Introduction

Materials and methods

Synthesis

Coating

Batch tests

Solution preparation

Adsorbent preparation

Tests

Results and discussions

Synthesis

Effect of coating time and adsorbent doses

Effect of contact time (Kinetic adsorption test)

Effect of pH

Effect of saline water

AC adsorption

Effect of regeneration method

Adsorption isotherms

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

References