Graphene oxide coated quartz sand as a high performance adsorption material in the application of water treatment

Article abstract of DOI:10.1039/c4ra11430b

In order to increase the water treatment performance, the quartz sand filter medium was improved by surface modification using a 3-aminopropyltriethoxy silane coupling agent (KH550), then GO was grafted to the surface of the sand though the chemical reaction between the functional groups. The interfacial interactions between the quartz sand surface and GO was studied. Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses showed that the GO binds strongly to the quartz sand surface. Scanning electronic microscopy (SEM) observation showed that a thin GO layer was formed on the surface of the modified quartz sand. The modified sand was used as a sorbent for the removal of turbidity, organic matter, Cd(ii) and Pb(ii) ions from large volumes of aqueous solutions. The results indicate that the GO plays an important role in improving the water treatment performance of a quartz sand filter.

Full text of DOI:10.1039/c4ra11430b

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PAPER
Graphene oxide coated quartz sand as a high
performance adsorption material in the application
of water treatment
Cite this: RSC Adv., 2015, 5, 8037
ab
Wenjun Hou,ab Yimei Zhang,
Tong Liu,c Hongwei Lud and Li Heabc
*
In order to increase the water treatment performance, the quartz sand filter medium was improved by surface
modification using a 3-aminopropyltriethoxy silane coupling agent (KH550), then GO was grafted to the surface
of the sand though the chemical reaction between the functional groups. The interfacial interactions between
the quartz sand surface and GO was studied. Fourier-transform infrared (FTIR) spectroscopy and X-ray
photoelectron spectroscopy (XPS) analyses showed that the GO binds strongly to the quartz sand surface.
Scanning electronic microscopy (SEM) observation showed that a thin GO layer was formed on the surface
of the modified quartz sand. The modified sand was used as a sorbent for the removal of turbidity, organic
matter, Cd(II) and Pb(II) ions from large volumes of aqueous solutions. The results indicate that the GO
plays an important role in improving the water treatment performance of a quartz sand filter.
Received 29th September 2014
Accepted 19th December 2014
DOI: 10.1039/c4ra11430b
www.rsc.org/advances
Following the discoveries of fullerene and carbon nanotubes
(CNTs) in earlier decades, the emergence of graphene with its
1. Introduction
combination of extraordinary physical properties has opened
up an exciting new eld in the science and technology of two
dimensional (2D) nanomaterials.14 Graphene's unique struc-
tural elements, including being a single two-dimensional and
extensively conjugated, endow it with advantageous thermal
conductivity,15 unique electric16 and mechanical properties,17
excellent mobility of charge carriers.18 Research has revealed
that graphene has exhibited better adsorption behavior for
heavy metal ions and uoride than nanotubes.19,20 Unlike
carbon nanotubes, which require special oxidation processes to
introduce hydrophilic groups to improve metal ion sorption,
the preparation of graphene oxide (GO) nanosheets from
graphite using Hummers method introduces many oxygen-
containing functional groups such as –COOH, –C]O, and
–OH, on the surfaces of GO nanosheets. These functional
groups are essential for the high sorption of heavy metal ions.
Natural quartz sand (NS) is widely used for processes of the
magnitude of municipal water supplies to small domestic water
lters, particularly as packed bed lters. History, affordability and
the granular nature of sand that forms lter beds have popularized
sand-ltration (SF). Indeed, early Indian and Greek writings dating
back 6000 years refer to sand and gravel-ltration as means to
securing clean water and currently are a water purication process
endorsed by the World Health Organization. Of the two broad
classications of SF, ne-SF has higher retention of pathogens,
organic matter, and heavy metal ions but has low throughput.
Although the production rates are higher for the more popular
coarse-SF, the absence of functionality and nanostructures limit
pathogen, organics and heavy metal ions retention.21–23
Pollution of freshwater systems by industrial and agricultural
waste has become one of the key environmental problems.1
Among all water contaminations, organic matter and heavy
metals ions can cause severe health problems in humans and
animals.2 For example, excessive Pb2+ could lead to a wide range
of health problems, such as nausea, convulsions, coma, renal
failure, cancer, and negative effects on metabolism and intelli-
gence.3,4 Excessive Cd2+ could also cause a range of diseases,
including hemochromatosis, gastrointestinal catarrh, cramps in
the calves, skin dermatitis brass chills5 and Wilson disease.6 Many
methods or technologies such as chemical precipitation,
membrane processes, ion exchange, electrolysis, adsorption
processes have been developed to remove the contaminations
from wastewater.7–11 However, each method has been found to be
limited by cost, complexity, efficiency as well as by secondary
waste.2 Among these technologies, adsorptive removal of organic
matter and heavy metals is widely applied because this method is
easy to handle, relatively low cost, and effective at low concen-
tration.12 Especially, the developments of nanoscience and bio-
logical science for organic matter and heavy metal removal have
received remarkable attention due to their special features.13
aSuzhou Research Academy of North China Electric Power University, Suzhou, Jiangsu,
China. E-mail: Yimei.zhang1@gmail.com; Fax: +86-0512-67332655; Tel: +86-0512-
67332668
bEnvironmental Research Academy of North China Electric Power University, Beijing,
China
cSchool of water resource and electric power, Qinghai University, Xining, Qinghai,
810016, People's Republic of China
dCollege of Renewable Energy, North China Electric Power University, Beijing, 102206,
China
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In Zhao's study,24 few-layered GO nanosheets were synthe- quartz sand were purchased from Sinopharm Chemical Reagent
sized from graphite using the modied Hummers method, and Co., Ltd.
were used as sorbents for the removal of Cd(II) and Co(II) ions
from large volumes of aqueous solutions. The results indicated
2.2 Synthesis of acyl chloride-functionalized GO (GO–Cl)
that abundant oxygen-containing functional groups on the
surfaces of GO nanosheets played an important role on Cd(II) and
Co(II) sorption. Gao25 demonstrated a simple technique for
conversion of regular ltration sand into “core–shell” GO coated
sand (GO–sand) granules by assembling water dispersible
graphite oxide on sand grains. The results showed that the
nanostructured GO-coated sand retains at least 5 fold higher
concentrations of heavy metal and organic dye than pure sand. In
this study, assembly process essentially consists of physical
mixing of the water dispersible GO colloids with sand, followed
by a mild heat treatment that causes the nanosheets to adhere to
each other over the sand surface, likely through van der Waals
interaction. Due to the combination of graphene and sand is
simply the physical effect, so in the process of water treatment,
graphene is easy to fell off the sand cause secondary pollution.
In order to solve the problem mentioned above, the surface
modication of natural quartz sand was made by using 3-amino-
propyltriethoxy silane coupling agent (KH550), then the surface
functional groups of quartz sand reacted with modied GO, the
color has gone from white to black (Fig. 1). So GO rmly planted on
the surface of quartz sand, will not fall off and have secondary
pollution. Through a series of experiments show that the GO
coated sand (GOS) granules has a strong adsorption performance
for organic matter and heavy metal ions.
GO was fabricated from graphite powder (GP) by oxidation with
KMnO4 in concentrated H2SO4, followed by hydrolysis, washing
and centrifugation, according to the modied Hummers'
method.26,27 This procedure has been conrmed by AFM to yield
GO sheets with majority of monolayer dispersed structures.
GO–Cl was prepared by reacting GO with SOCl2.28 As shown
in Fig. 2, 50 mg GO was rst sonicated in 20 mL of anhydrous
DMF to form a homogeneous suspension, to which 50 mL SOCl2
was added dropwise at 0 ꢀC. GO–Cl was prepared by stirring at
0 ꢀC for 2 h, and keeping stirring for 36 h at 70 ꢀC. The obtained
GO–Cl was collected by ltration through a PTFE membrane.
2.3 Preparation of quartz sand lter
The quartz sand lter with size fractions of 0.55 to 0.83 mm was
obtained by dry-sieving through stainless steel sieves. The lter
medium was boiled in deionized water for 30 min followed by
washing several times with deionized water until the washed
water no longer seemed turbid. Before being used, the cleaned
lter medium was dried at 110 ꢀC for 12 h. This procedure
substantially eliminates the effect of any impurity on the lter
medium surface.
2.4 Preparation of KH550-coated sand (KHS)
2. Experimental
Synthesis route of KHS is show in Fig. 3. Firstly, 15% KH550 was
added to alcohol–water solution to hydrolyze it sufficiently. The
volume proportion of alcohol and water was 2 : 1. Secondly, the
prepared quartz sand was added into above solution. The
system was stirred and heated up to 70 ꢀC using a water bath for
2.1 Materials
Graphite powder (20 mm), concentrated H2SO4, SOCl2, KMnO4,
N,N-dimethylformamide
propyltriethoxy silane coupling agent (KH550) and natural
(DMF),
pyridine,
3-amino-
ꢀ
15 min followed by dried for 2.5 h at 110 C. Thirdly, the reac-
tion product was immersed in deionized water for 24 h and
washed several times with deionized water to eliminate the
inuence of physical adsorption. Lastly, the surface modied
ꢀ
lter was obtained by dried at 110 C for 12 h.
2.5 Preparation of GOS
KHS was mixed with GO–Cl in pyridine, then stirring at 0 ꢀC for
12 h. The coated sand was washed with distilled water until the
ꢀ
runoff was clear, dried at 60 C, and stored in capped bottles.
Fig. 1 Photographs of NS (a) and GOS (b). Schematic diagram of the
experimental set-up (c).
Fig. 2 Synthesis route of GO–Cl.
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pretreated GO (80 mg) were stirred in 50.00 mL of 0.05 M
aqueous NaOH solution under argon for 48 h. The mixture was
ltered through a 1.2 mm pore-size membrane and washed
with deionized water until the ltrate was neutral. The
combined ltrate and washings were titrated with 34.50 mL
of 0.05 M aqueous HCl solution to reach the neutral point
(pH ¼ 7.00), as monitored by a pH meter, and thus the
content of acidic sites in the GO is calculated to be
0.78 mmol. The carbon content of the GO is estimated to be
6.67 mmol by assuming that the GO is solely composed of
carbon, and thus the molar ratio of the acidic sites in the GO
sample is 11.6%.
In our experiments, GO–Cl was rstly synthesized from GO
(Fig. 2), because the acyl chloride on the surface of GO are
highly reactive. It is easy to react with modied sand, so GO can
binds strongly to the surface of quartz sand (Fig. 4). The
molecular formula of KH550 modier and its chemical changes
during hydrolysis are shown in Fig. 3a. During hydrolysis of the
silane, the SiOC2H5 group will transform into Si–OH. We
hypothesize that the surface modication mechanism is carried
on the mode given in Fig. 3b.29
Fig. 3 The action mechanism of KH550 on the surface of quartz sand.
2.6 Column test on NS and GOS
AFM is a direct method to characterize the morphologies and
the thickness of GO. Fig. 5 shows the tapping mode AFM images
of GO. The thickness of GO is about 0.8 nm (Fig. 5b), which
agrees with other researchers' reports,30,31 suggesting the
complete exfoliation of graphite oxide into individual GO sheet.
The FTIR spectra of NS, KHS and GOS are shown in Fig. 6.
From Fig. 6a we can see the asymmetry stretching vibration
and deformation vibration of Si–O–Si at 1200 cmꢂ1 to
1100 cmꢂ1 and 472 cmꢂ1, which indicated the formation of
quartz sand lter. From Fig. 6b we can see that the absorption
peaks at 2935 cmꢂ1, 2893 cmꢂ1 and 3320 cmꢂ1 ascribed to
asymmetry stretching vibration, symmetry stretching vibra-
tion of C–H and stretching vibration of N–H, respectively,
which demonstrates that KH550 exists in quartz sand surface.
In the spectra of Fig. 6c, the characteristic absorptions of
amide II (at 1550 cmꢂ1) were clearly observed, it can be
concluded that the GO has been incorporated into the surface
of sand. Moreover, the physical adsorbed KH550 and GO were
washed with deionized water for several times in surface
modication process, so the physical adsorption between
quartz sand and GO can be eliminated.
The Raman spectra of GOS sample in Fig. 7 show the char-
acteristic peaks located at 1339 and 1598 cmꢂ1 corresponding to
the D and G bands of graphene sheets, respectively,32 while
barely any features are obtained from NS spectrum.
In this work, XPS analysis of the quartz sand before and aer
surface modied were employed to conrm the interface
bonding way between graphene and the quartz sand surface.
The results of the XPS analyses are shown in Fig. 8. Table 1
shows the detailed data corresponding to Fig. 8. It is shown in
Fig. 8 that nitrogen and carbon content are not present at the
surface of the unmodied quartz sand. In contrast, KHS
A mini-pilot-scale equipment was constructed and used in this
study (Fig. 1c). For the column tests, the adsorption column
(5.2 mm diameter ꢁ 310 mm long) was lled with NS (Fig. 1a)
or GOS (Fig. 1b), and the feed solution was owed through
the column at controlled ow rate, the eluted solution
was continuous collected every 1 hour and the ow rate is
1 mL minꢂ1
.
2.7 Characterization
Atomic force microscope (AFM) observation of GO sheets was
recorded using
a Vecco Digital Instrument Multimode
V scanning probe microscope, in which samples prepared by
spin-coating sample solutions onto freshly exfoliated silicon
substrates. Fourier-transform infrared (FT-IR) spectra were
recorded on a ProStar LC240 (Varian, USA). Scanning electron
microscopy (SEM) images were taken on a Hitachi S-47000
eld-emission SEM system. Thermal gravimetric analysis
(TGA) was carried out on a Perkin-Elmer Pyris 6 TGA instru-
ment at a heating rate of 20 ꢀC minꢂ1 under nitrogen ow.
Raman spectra were collected on a Jobin-Yvon Raman spec-
troscope equipped with a 514 nm laser source (HR800,
France). The turbidity was measured with a turbidimeter
Hanna model HI93703. Dissolved organic carbon (DOC) was
measured with TOC analyzer (TOC-VCPH, Shimadzu, Japan).
UV254 was measured with spectrometer (UV754, CANY, China).
The concentrations of Cd2+ and Pb2+ were determined by
ICP-MS.
3. Results and discussion
3.1 Synthesis and characterization of GO and GOS
The concentration of carboxylic acid groups in the GO was contains nitrogen and carbon. Furthermore, there is a signi-
estimated by acid–base titration. Before titration, the GO were cant depletion in oxygen content and a slight decrease in silicon
stirred in water for 2 days under argon and collected via ltra- content on the KH550 modied quartz sand surface in
tion to remove acid residue. In a typical titration experiment, comparison to the unmodied quartz sand, while for GOS, the
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Fig. 4 Schematic illustration of the fabrication of GOS nanocomposite coatings.
Fig. 5 AFM height image of GO on silicon wafer deposited from
aqueous solution.
Fig. 7 Raman spectra of NS and GOS.
Fig. 6 FTIR spectra of NS (a), KHS (b) and GOS (c).
carbon content increased to 52.43. This is further affirmed that
the GO is well bind to the surface of quartz sand.
Fig. 9 corresponds to SEM images of NS, KHS and GOS.
Specically, Fig. 9a shows the extremely rough and uneven of the
unmodied quartz sand surface, while the KH550 modied quartz
sand surface possesses comparatively a atter surface (Fig. 9b) due
Fig. 8 XPS spectra of NS, KHS and GOS.
TGA curves of the NS, KHS and GOS are shown in Fig. 10. The
to a thin layer of KH550 graed on the surface of quartz sand lter. TGA data for GOS showing a weight loss of ꢃ8.0% contrasted with
In the Fig. 6c and d, we can see that a lot of graphene coating on the that of NS. Comparatively, 7% weight loss of KHS could be
surface of quartz sand and more tiny pores appear on its surface. observed, which can strongly prove that GO has graed on the
This result is accordant with the analyses results of XPS and FT-IR. sand surface.
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Table 1 Results of elemental analysis by X-ray photoelectron spec-
troscopy on the modified and unmodified quartz sand filter
dissolved organic fraction. Particulate organic matter (POM)
could be easily removed even through the sand-ltration;
whereas dissolved organic matter (DOM) is one of the major
concerns in drinking water treatment due to its difficulty to be
removed. During the experiments, the raw water had average
dissolved organic carbon (DOC) and UV254 concentrations of
5.398 mg Lꢂ1 and 0.248 cmꢂ1, respectively. As shown in Fig. 12a
and b, DOC was reduced to 2.0 mg Lꢂ1 through the GO treat-
ment and about 60% of the raw water UV254 was removed at the
beginning. Furthermore, it could be observed that the removal
efficiency of the GOS for DOM was rather stable during the
6 hours of operation. Therefore, it was reasonable to infer that
the DOM was removed mainly through graphene on the sand.
Aer experiment, the surface morphology of GOS has been
included in Fig. 13. It is obvious that the GO didn't peel off from
the sand. In addition, GO has a hydrophilic surface with plenty
of oxygen-containing groups, and good dispersion in water.
Both hydrogen bonding and p–p interaction were thought to be
responsible for the adsorption of DOM on GO.
Element
NS
KHS
GOS
Si (%)
O (%)
C (%)
N (%)
29.25
70.75
—
20.56
47.76
31.68
0.22
12.36
35.21
52.43
0.14
—
3.2.3 Adsorption of Cd2+, Pb2+, Cu2+ and Zn2+ by NS and
GOS. In order to nd out whether GOS sample could effectively
adsorb heavy metal ions, adsorption studies were extended to
Cd2+, Pb2+ Cu2+ and Zn2+. Comparative study of NS and GOS
were experimented (Fig. 14). Aqueous solutions (400 ppb) of
Cd2+, Pb2+ Cu2+ and Zn2+ were prepared by dissolving Pb(NO3)2,
Cd(NO3)2, CuSO4 and ZnCl2 in ultrapure water at 25 ꢀC and pH
5, further diluted to the required concentration just before
usage. The results revealed that both NS and GOS exhibit high
absorption capacity for Cd2+, Pb2+, Cu2+ and Zn2+ when the
inuent volume is less than 50 mL. It is obvious that the
adsorption capacity of GOS was higher than that of NS during
the experiment.
3.2.4 Recyclability of GOS. As an effective absorption
material, the absorption recyclability has been studied. The
above metal ions adsorbed GOS was thoroughly rinsed with
water and dried in a vacuum. Then it was put into a 10 mL nitric
acid solution at pH 1 for desorption. Aer two days, the metal
ions was completely desorbed from the GOS, it was removed
from the acid solution, thoroughly rinsed with water and dried
in the freeze drier. The regenerated GOS was then subjected to
Fig. 9 SEM micrographs of NS (a), KHS (b) and GOS (c and d).
3.2 Effectiveness of the GOS for pollutants removal
3.2.1 Turbidity removal. In order to investigate the perfor-
mance of the new material in the eld of water treatment, its
removal to turbidity was studied (Fig. 11). The turbidity of raw
water is 21.03 NTU. The result showed that as compared with NS
lter media, the GOS is more efficient in treatment of water. The
turbidity removal efficiency of GOS is about 38% higher than NS.
This may be because of the introduction of GO make the surface of
quartz sand has more holes and raise the dirt holding capacity.
3.2.2 Organic matter removal. Total organic matter in
water might be classied into particulate organic fraction and
Fig. 10 TGA curves of the NS, KHS and GOS.
Fig. 11 Turbidity removal efficiency of NS and GOS.
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Fig. 12 Removals of DOC (a) and UV254 (b).
Fig. 15 Cd2+, Pb2+, Cu2+, and Zn2+ removal efficiency of used GOS.
attributed primarily to two factors: a decrease in the number of
binding sites aer each desorption step and the degradation of
the GOS themselves.
Fig. 13 SEM micrographs of GOS (after water treatment).
4. Conclusions
In this work, we synthesized graphene–sand composites using
acyl chloride-functionalized GO and modied quartz sand, GO
rmly combined in the surface of quartz sand by chemical
bond. This creates a new kind of porous carbon-based multi-
functional adsorption material. Through a series of adsorp-
tion test, we conclude that the nanostructured GO coating can
signicantly increase the retention of turbidity, organic matter
and heavy metals over the parent sand granules. This kind of
technique combine together the excellent function of nano-
structures material with the traditional quartz sand, and have
very widely applied foreground in the water treatment.
Fig. 14 Cd2+ (a), Pb2+ (b), Cu2+ (c), and Zn2+ (d) removal efficiency of
NS and GOS.
Acknowledgements
This research was supported by the plan of basic applied study
of Suzhou (SYG201417) and the Fundamental Research Funds
for the Central Universities (JB2013115).
repeated adsorption cycles as described above to investigate the
recyclability of the GOS. The results in Table 2 and Fig. 15 show
that the absorption ability of GOS changed obviously during the
continuous use. The decrease in adsorption ability can be
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