Preparation of nanocarbon modified vermiculite ceramics with excellent adsorption capacity

Nanocarbon modified vermiculite porous ceramics with hydrophobicity were prepared using vermiculite as the starting materials and polyethylene terephthalate (PET) as the carbon precursor via the catalytic reaction method. The microstructures, hydrophobicity, and adsorption capacity of as-prepared porous ceramics were characterized by scanning electron microscope, water contact angle testing, and weight gaining, respectively. The results revealed that the nano-carbon generated from the pyrolysis of PET formed on the surface of the pores in ceramics. The water contact angle of as-prepared modified porous ceramics with 82% porosity was up to 123 °. The as-prepared porous ceramics showed highly efficient adsorption to oils and were very durable, and would be excellent candidate materials to separate the oil from the water/oil mixture.


Introduction
In recent years, with the rapid development of economic society, there has been a sharp increase in oil spills and output of oily industrial wastewater worldwide, which has become a significant concern for offshore oil exploration, production, and transportation, posing a tremendous threat to the ecosystem and human health. Many research works have been conducted to remove organic liquids or oils from water [1][2][3][4][5]. Chemical, biological, and physical methods are commonly used to deal with this issue, such as solidifying oils [6], in situ burning [7,8], using particular microorganisms to degrade pollution [9], and adsorbing oils by sorbent matrix [10,11]. Among them, adsorption methods are suggested as green and efficient strategies [12]. Sorbent materials, such as organic materials [13,14], polymer-based composites [15][16][17][18], synthetic polymers [19], and inorganic minerals [20] are widely employed due to their environmental friendliness and convenient operation. Despite the promising results obtained, the works reported still suffered from the following disadvantages: (1) The micronano structure on the surface of the final product is easily damaged in a harsh environment, making it nonrecyclable under a harsh environment [21,22] (2) The sponge and cotton fabrics as starting materials were low strength and difficult to recycle, and (3) most of the organic modifiers are not environmentally friendly.
Inspired by the structures of spider silk, lotus leaves, etc, many materials with excellent hydrophobic characteristics were prepared. It is well known that ideal hydrophobic depends in the following two ways: (1) low surface energy, and (2) high surface roughness [23][24][25]. Nanocarbon is one of the low surface energy and high surface roughness materials [26,27].
Porous ceramics could be employed as efficient adsorption materials due to their high porosity and corrosion resistance [28][29][30]. However, these ceramics are difficult to use as separation materials because of their hydrophilic features directly. Meanwhile, the amounts of waste plastics increased drastically due to the excessive application of plastic products every year. As one of the most common plastics, soft drink bottles have been considered major municipal solid waste [31]. The disposal of plastic garbage has been attracting more attention. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
In this work, porous ceramics modified with hydrophobicity were prepared using vermiculite as the starting materials and polyethylene terephthalate (PET) as the carbon precursor via the catalytic reaction method, which has the following highlights: (1) Vermiculite is nontoxic and readily available in large quantities at low cost, and does not cause any environmental problems after use, thus can be considered as an ideal raw material. By combining layered structure of the vermiculite and large pores generated from the vibrating&casting molding operation, nanocarbon modified vermiculite porous ceramics can be prepared as a potential 3D sorbent. (2) Asprepared porous ceramics possess better mechanical strength and better abrasion resistance more than their polymer-based counterparts. Moreover, they can be easily manufactured in a sizeable monolithic form and used safely in harsh conditions. (3) Compared to polymer products, porous ceramics can be used at a wider temperature range.

Preparation of modified porous ceramics
The porous matrix was prepared with expanded vermiculites as raw materials, bound by sodium silicate through the vibration molding. In a typical preparation process, 10 g of vermiculite powder, and 50 g of sodium silicate were stirred for 10 min in a mechanical mixer. Then, the mixtures were shaped in a mold (f30 × 30 mm) by vibrating for 10 min. After 24 h of drying at room temperature, the specimens were demolished and dried for another 24 h at 110°C.
The waste plastic bottles were first cut into small slices, and then these slices and the as-prepared green body were dipped respectively into the Fe(NO 3 After been dried, the porous ceramics embedded with the waste plastic slices, were placed into an alumina crucible and coated with a lid. Thus, the modified porous ceramics were fabricated after being heated for two h at 700°C under flowing N 2 .

Characterization of porous ceramics
In the present work, the adsorption properties of modified porous materials were carried out in accordance with the weight change. After the specimens were dipped in the paraffin oil for 5 min, the specimens were taken out, and weighted until no residual oil drops were found on the specimen surface. Each adsorption procedure was performed three times to obtain the average values. The adsorption capacity Q (g g −1 ) of the specimens was carried out according to equation (1): where m 0 (g) is the initial mass of specimen, m (g) is the mass of specimen after adsorption. To identify the recyclability of the specimen, the specimens, after adsorbing paraffin oil, were ignited in the air to remove the oil, and then the adsorption test process was repeated. The residual adsorption capacity R was performed according to equation (2): where Q t is the adsorption value of the specimen after t times, and Q 0 is the original adsorption value of the specimen. Furthermore, as an example, the separation properties of as-prepared porous materials for water and oil were carried out using mixed water/paraffin oil. First, the deionized water was mixed with paraffin oil stained with Sudan Red III. Then, the modified porous materials were placed into the mixtures, and a series of photographs were recorded to illustrate the oil adsorption process.
According to the Archimedes method, apparent porosity, as well as bulk density, was calculated. The morphologies and microstructures were observed by scanning electron microscope (SEM, JSM-6610m, Japan).

Results and discussion
3.1. Microstructure of the specimens Expanded vermiculites are suitable substrates for producing adsorption materials because of their chemical inertness and high porosity. Figure 1 shows the typical microstructures of vermiculites. As shown in figure 1(a), many pores could be observed in the expanded vermiculite particles. Figure 1(b) presents the microstructure of as-prepared porous materials, suggesting that the porous structure of expanded vermiculites remained after molding and heating. The apparent porosity and the bulk density of as-prepared porous materials were 82% and 0.48 g cm −3 , respectively. Figure 2 shows the microstructures of modified porous ceramics. It was found that many microwormlike rods formed on the inner walls of pores in the specimens ( figure 2(a)). High magnification (the square area 1 in figure 2(a)) shows that the worm-like rods have several hundred nanometers in length, about 100 nanometers in width (as shown in figure 2(b)). The square area 2 in figure 2(a) showed the clustering micro-wormlike rods and their EDS analytic result shown in figure 2(d) illustrated that the micro-wormlike rods were composed of C, Si, Al, and other elements. Polyethylene terephthalate could decompose at about 380°C [32]. In the present work, it could be illustrated that polyethylene terephthalate decomposed by the pyrolysis process and deposited on the inner wall of pores to form carbon nanorods with the Fe catalyst at 700°C.

The hydrophobicity of modified porous materials
The hydrophobicity of modified materials was analyzed by testing the water contact angle of the specimens. Figure 3 presents the digital pictures of a drop of water dripping onto the modified materials. For the specimen  without Fe catalyst, its water contact angle was 91°(figure 3(a)). Figure 3(b) shows the digital pictures of the specimen with 0.5% Fe(NO 3 ) 3 ·9H 2 O, which showed the water contact angle was 122°. When the concentration of Fe(NO 3 ) 3 ·9H 2 O was 1.0% and 2.0%, the water contact angle of specimens was 123°and 93°, respectively. Compared to specimens without catalyst, all of the water contact angles of specimens with catalyst were increased. These results suggested that the water contact angle increased obviously as a result of the introduction of the Fe catalyst. Thanks to the formation of nanocarbon in the porous ceramics, the modified porous ceramics were hydrophobic. As we know, the Fe(NO 3 ) 3 ·9H 2 O could discompose and be reduced to Fe in the reduction condition [33]. These results confirmed that Fe played an important catalytic role during nanocarbon formation, and the optimum concentration of Fe(NO 3 ) 3 ·9H 2 O was 1.0% in the present work. It could also be noticed that the contact angle of the specimen with 2.0% Fe(NO 3 ) 3 ·9H 2 O decreased to 93°. The reason for the reducing contact angle may be the aggregating of catalyst particles, which deteriorates the catalytic effect of the catalyst.
Under the same experimental conditions, the effects of the catalysts of Co and Ni on the hydrophobicity of specimens were also studied. Figure 4 showed the digital images of specimens with 0.5%, 1.0%, and 2.0% Co(NO 3 ) 2 ·6H 2 O, respectively. The contact angles of the specimens were 95°, 103°, and 124°. Those data suggested that the optimal concentration of Co(NO 3 ) 2 ·6H 2 O was 2.0%. Figure 5 showed digital pictures of specimens with 0.5%, 1.0%, and 2.0% Ni(NO 3 ) 2 ·6H 2 O, respectively. The contact angles of the specimens were 111°, 113°, and 100°. It suggested that the optimal concentration of Ni(NO 3 ) 2 ·6H 2 O was 1.0%.
According to the results presented above, it could be found that the Fe catalyst exhibited the best catalytic role. The specimen owned the best hydrophobicity (water contact angle 123°) due to the use of a small amount of Fe (1.0%). Catalytic synthesis methods have recently received more attention as a result of lower reaction temperature, high efficiency, and less energy consumption [34][35][36][37][38][39][40]. The study by Wang et al [41] shows that ferric nitrate transformed into Fe 3 O 4 at 673 K, and into metallic Fe and Fe x C carbide at 873-1273 K. The optimal weight ratio of Fe catalyst to phenol resin for CNT growth was 1.00 wt%, and the optimal temperature was 1073 K. Density functional theory (DFT) calculations suggest that Fe catalysts facilitate the CNT growth by increasing the bond length and weakening the bond strength in C 2 H 4 via donating electrons to the C atoms in it. Wang et al [42] investigated the effects of Co, Fe, and Ni catalysts generated from their nitrate precursors on the formation of carbon nanotubes (CNTs) from the pyrolysis of phenolic resin. The molecular dynamics calculations show that the Fe nanocluster has the highest melting point among the above three nanoclusters and can retain its integrity at 1280 K, making Fe the best catalyst at high temperatures. These results revealed that the reason for the best catalytic performance of Fe catalysts is the highest adsorption energy of Fe catalysts among the three catalysts, which illustrated a strong interaction between the catalyst and the reactants exhibiting increased bond length and weakened bond strength. Meanwhile, the high melting point that retains its integrity until 1280 K maybe the other reason for its best catalytic performance.

Adsorption properties of modified porous ceramics
The modified porous ceramic specimen was placed into the oil/water mixture. A series of digital photos (figures 6(a)-(f) was recorded to illustrate the dynamic separation process. The entire separation progress was completed in 1 min. These results revealed that the modified porous ceramics could separate the paraffin oil from oil/water mixture rapidly. It was also found that the modified porous ceramics floated on the water, making it possible to utilize the modified porous ceramics to sorbent oil/organic liquid floating on lakes or seas.
The adsorption capacity of modified materials was measured according to the weight gain refered to equation (1). The adsorption capacity of modified materials for paraffin oil was tested to be 0.640 g/g. Compared to the inorganic traditional sorbent materials, the adsorption capacity increased by about 4-20 times (table 1). As presented in figure 7, the modified materials reached saturated adsorption for paraffin oil in 30s. These results suggested that the as-prepared materials exhibited excellent adsorption efficiency for oil. The reasons why nanocarbon modified vermiculite ceramics have excellent adsorption performance may be the followings: (1) Vermiculite ceramics have more pores than the bentonite and diatomite mentioned in [43,44], especially largesize pores coming from the process of ceramics preparation and their own porous structure, which the oil molecule can enter into. (2) The surface of vermiculite ceramics was modified with nanocarbon.
The adsorption capacity and recyclability of materials must be considered simultaneously for practical application. With reference to the methods reported by Han [45], combustion was the appreciated way to regenerate the as-prepared modified porous ceramics. A recyclability test was carried out by using paraffin oil as an organic liquid. The residual adsorption R was carried out according to equation (2). The adsorption capacity Q of as-prepared specimen after 1-5 cycles was 0.640, 0.610, 0.600, 0.600, and 0.600 g g −1 , and the residual adsorption ratio R was 100, 95.3, 93.7, 93.7 and 93.7%, respectively (figure 8). The residual adsorption ratio R of modified porous ceramics was found up to be approximately 93.7% even after five cycles. These results revealed that the as-prepared materials had better recyclability.

Conclusions
Vermiculite-based porous ceramics modified by nano-carbon were prepared by a catalytic pyrolysis reaction using waste plastic bottles as carbon procurers. The modified porous ceramics with high porosity presented very effective adsorption for oil. The adsorption capacity of modified porous ceramics for the paraffin oil was up to   0.660 g g −1 , which was 4-20 times higher than other inorganic traditional materials. These modified porous ceramics exhibited excellent recyclability, and their residual adsorption ratio was up to about 93.7% after five cycles. The as-prepared materials might be a good candidate material to separate oil from an oil/water mixture.