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Xi, Yunfei, Mallavarapu, Megharaj, & Naidu, Ravendra(2010)Preparation, characterization of surfactants modified clay minerals and ni-trate adsorption.Applied Clay Science, 48(1-2), pp. 92-96.

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https://doi.org/10.1016/j.clay.2009.11.047

1

Preparation, characterization of surfactants modified

clay minerals and nitrate adsorption

Yunfei Xi1,2, Megharaj Mallavarapu1,2 and Ravendra Naidu1,2

1 Centre for Environmental Risk Assessment and Remediation (CERAR), University

of South Australia, Mawson Lakes, SA 5095, Australia

2 Cooperative Research Centre for Contamination Assessment and Remediation of the

Environment (CRC CARE), University of South Australia, Mawson Lakes, SA 5095,

Australia

Abstract

Organic surfactants modified clay minerals are usually used as adsorbents for

hydrophobic organic contaminants remediation; this work however has shown

organoclays can also work as adsorbents for hydrophilic anionic contaminant

immobilization. Organoclays were prepared based on halloysite, kaolinite and

bentonite and used for nitrate adsorption, which are significant for providing

mechanism for the adsorption of anionic contaminants from waste water. XRD was

used to characterize unmodified and surfactants modified clay minerals.

Thermogravimetric analysis (TG) was used to determine the thermal stability and

actual loading of surfactant molecules. Ion chromatography (IC) was used to

determine changes of nitrate concentration before and after adsorption by these

Corresponding Author Telephone: +61 8 83026232, Email address: yunfei.xi@unisa.edu.au (Yunfei Xi), Postal address: X-building, University of South Australia, Mawson Lakes, SA 5095, Australia

2

organoclays. These organoclays showed different removal capacities for anionic ions

from water and adsorption mechanism was investigated.

Keywords: Anion; adsorption; organoclay.

1. Introduction

Clay minerals have been widely used in a range of applications because of

their high cation exchange capacity, swelling capacity, high specific surface area, and

consequential strong adsorption capacity (Bailey et al., 1999; Du and Hayashi, 2006;

Sen Gupta and Bhattacharyya, 2006). Bentonite is made up primarily of

montmorillonite. Other clay minerals that are included in this study are kaolinite and

halloysite. Applications of clay minerals are extended to anionic contaminants

particularly inorganic oxyanions, for example, nitrate, chromate and arsenate which

are toxic to human and wildlife even at very low concentration. However, these

anions can be repelled by the negative charges on clay minerals’ surface, so natural

unmodified clay minerals are ineffective adsorbents for these contaminants. For this

reason, modification using an additive has shown enhanced anions retention capacity.

In addition, the optimum modification method should impart high selectivity towards

anions. There are some studies that have indicated clay minerals when modified by

organic surfactants will become suitable materials for anions retention (Kaufhold et

al., 2007; Krishna et al., 2000; Li and Bowman, 1998; Li and Bowman, 2001). Thus,

the surface modification of clay minerals has received great interest, for example, ion

exchange of the inorganic cations with organic cations usually with quaternary

ammonium compounds can change the surface properties (Mortland et al., 1986). The

intercalation of cationic surfactants not only changes the surface properties from

3

hydrophilic to hydrophobic but also greatly increases the anions adsorption capacity

especially when surfactant loading exceeds the CEC of clay. The resulting adsorption

of surfactant molecules via hydrophobic bonding (Lee and Kim, 2002; Xu and Boyd,

1995) and the positive charge of ammonium will attract anions. Such surface property

modification is of importance for extended organoclays applications. In particular, at

the same time, the hydrophobic nature of the organoclay implies that the material can

be used as a filter material for water purification (Beall, 2003). Compared to pure

organic surfactants, these organoclays also exhibit a remarkable improvement in

properties which includes increased strength and heat resistance, decreased gas

permeability and flammability etc. (Dultz and Bors, 2000). All of these applications

and improvements mentioned above strongly depend on the structure and properties

of the organoclays. So understanding the structure and properties of organoclays is

essential for their applications.

Organoclays have been extensively investigated for hydrophobic contaminants

immobilization, however, the objective of this study is to investigate the hydrophilic

anionic contaminants adsorption on organoclays, and in particular this study

investigated the properties of these clays modified with different

hexadecyltrimethylammonium bromide dosages and their performance on nitrate

adsorption. X-ray diffraction technique was used to investigate the phases and basal

spacings of these materials. Thermogravimetric analysis (TG) was used to probe the

microenvironment and packing arrangement of organic surfactant within the

organoclays and to calculate the real surfactant loading within organoclays. Such a

study is of high importance for understanding the structure, properties, and potential

applications of organoclays for anionic contaminants removal from waste water.

4

2. Experimental

2.1. Materials and preparation

The natural bentonite was supplied by Pacific Enviromin Ltd, which originates

from Queensland, Australia. Kaolinite was supplied by Skardon River Kaolinite Pty,

Australia and halloysite was supplied by IMERYS tableware from New Zealand.

These clay minerals were passed through a 200 mesh sieve and used without further

purification. The cation exchange capacities (CECs) were 66.7, 9.8 and 10.0 meq/100

g respectively as determined by ammonia electrode method (Borden and Giese, 2001).

The surfactant used was hexadecyltrimethylammonium bromide labelled HDTMA

(CH3(CH2)15(CH3)3N+Br−, FW: 364.45) with a purity of 99% as supplied by Sigma-

Aldrich and it was used without any further purification. NaNO3 and NaBr were

supplied by BDH, both are in analytical purities.

The syntheses of surfactant modified clay minerals were undertaken by a

similar procedure as described previously by (Frost et al., 2008): The clarifying

surfactant solution was obtained when certain amounts of HDTMA were added into

hot distilled water. Then certain amounts of bentonite, kaolinite and halloysite were

added into the above mentioned solution respectively and the dispersions were stirred

slightly in order to avoid the yield of spume in an 80 °C water bath for 2 h. The

water/clay mass ratio was 10. Then the suspension was subsequently washed with

deionized water (MilliQ system) for 4 times. The moist solid material was dried at

60 °C and ground with an agate mortar. In our previous study (unpublished), good

sorption capacities for anions was shown on bentonite modified with 2 and 4 CEC of

surfactant, so the concentrations of HDTMA used in this study were also set at 2 and

4 CEC for all three clay samples. Bentonites modified with 2 CEC and 4 CEC

respectively with HDTMA were denoted as H-B-2CEC and H-B-4CEC, while

5

similarly, other organoclays were denoted as H-K-2CEC, H-K-4CEC, H-H-2CEC and

H-H-4CEC respectively.

Adsorption measurements were made by mixing nitrate solutions with

different organoclays at room temperature. About 0.2 g of organoclay was placed in

50 ml centrifuge tube containing 40 ml of anion solution at 100 mg/L, pH 5.4.

Solutions were shaken for 17 hrs on an end-over-end rotary shaker at room

temperature. The adsorbent was separated using a centrifuge at 4000 rpm for 15 mins.

Then the supernatant liquids were filtered through 0.45 µ Nylon membrane filters and

then 1 ml was removed and transferred to 1.5 ml vials for ion chromatography (IC)

tests. The amount of anions sorbed by organoclays was calculated by difference

between the initial and final solution concentrations.

2.2. Characterisation methods

X-ray diffraction (XRD) patterns were recorded using CuKα radiation

(monochromated with LiF) on a Diffraction Technology MMA diffractometer

operating at 35 kV and 28.2 mA (1 kW) with a 1° divergence slit and a 0.5°

antiscattter slit at a step size of 0.02°. The specific surface areas of samples were

measured by multipoint BET method on a Gemni 2380 surface analyser. Samples

were outgassed at 353 K for 48 hrs at about 10-4 Torr. Thermogravimetric analyses of

the clays and surfactant-modified clays were obtained using a TA Instruments Inc. TA

2950 high-resolution TGA operating at a ramp of 10 °C/min from room temperature

to 900 °C in a high-purity flowing nitrogen atmosphere (50 cm3/min). Approximately

30 mg of the finely ground sample was heated each time in an open platinum crucible.

Nitrate concentrations were measured using a DIONEX ICS 2000 ion

6

chromatography, which is equipped with an AS18 column. Synthetic NO3- solutions

were prepared by dissolving the appropriate amounts of NaNO3 in deionized water.

3. Results and discussion

X-ray diffraction (XRD): The modification of the bentonite by surface reaction with a

cationic surfactant through its incorporation into the structure can be followed through

the expansion of the clay mineral layer. XRD pattern of untreated QLD-bentonite

sample was shown in Fig. 1a. Similar to that found in our pervious study, two

reflections at 20.7 and 12.7 Å were observed. The later was attributed to the basal

spacing of this bentonite, whereas the former reflected a supercell or superlayer

resulting from the packing arrangement of neighbouring layers arising from different

layer charge densities (Xi et al., 2005). XRD patterns of organoclays prepared using

HDTMA with 2 CEC and 4 CEC surfactant concentration loadings were shown in Fig.

1b and Fig. 1c respectively. With the cation exchange by the cationic surfactant,

expansion of the bentonite layers occurred. As shown in Fig. 1b, only one dominant

reflection at 20.2 Å was observed corresponding to basal spacing with surfactant

molecule laying flat between clay mineral layers. While in Fig. 1c, two d(001)

spacings were observed at 35.7 Å and 18.5 Å respectively. The reason for the two

spacings can be attributed to the arrangement of the HDTMA surfactant molecule

within the clay mineral layers; the spacing of 18.5 Å was ascribed to the bentonite

expanded with HDTMA molecule laying flat between two layers, while 35.7 Å

spacing was attributed to the surfactant molecules at right angles to the clay mineral

surface. Kaolinite was characterized mainly by intense first and second-order basal

reflections at 7.1 Å and 3.6 Å (Fig. 2a). Halloysite can be intercalated with a

monolayer water molecules which will give out a basal spacing at around 10 Å

7

(Churchman, 1990; Joussein et al., 2007). However in this pattern, no water layer at

about 10 Å was observed. This halloysite showed similar XRD pattern as that of

kaolinite with reflections at 7.23, 4.42, 4.03, 3.56, 3.33, 2.55, 2.48, 2.34 and 2.28 Å,

while reflections at 4.03 Å and 3.33 Å were from impurities cristobalite and quartz

respectively. The HDTMA modified kaolinite and halloysite (shown in Fig. 2b, 2c, 2e

and 2f) did not show much change on XRD patterns compared to those of untreated

ones.

BET Surface area: The specific surface areas for bentonite, kaolinite and halloysite

were 20.1 m2/g, 30.9 m2/g and 26.5 m2/g respectively as measured by the BET

method (Brunauer et al., 1938); and the specific surface areas of prepared organoclays

were measured at 0.9 m2/g, 0.6 m2/g, 20 m2/g, 17.8 m2/g, 16.1 m2/g and 17.4 m2/g for

H-B-2CEC, H-B-4CEC, H-K-2CEC, H-K-4CEC, H-H-2CEC and H-H-4CEC

respectively. It was reported in a literature (Kukkadapu and Boyd, 1995) that when a

montmorillonite was modified with smaller sized surfactant such as

tetramethylammonium chloride (TMA), the surface area of prepared organoclay

increased comparing with that of the unmodified clay. But in this study, the results

showed that the surface area of clay decreased when modified with HDTMA which

has longer carbon chain, and with the increase of added surfactant dosage, generally

speaking, the surface area of prepared organoclay decreased, The decease of surface

area may be attributed to exchange sites which were satisfied by HDTMA with large

molecular size resulting in inaccessibility of the internal surface to nitrogen gas and

the blocking of the micropores in the HDTMA modified clays. It was in accordance

with conclusions drawn in literatures (Burns et al., 2006; Seki and Yurdakoc, 2005).

For HDTMA modified halloysites, similar surface areas were obtained on H-H-2CEC

and H-H-4CEC. In addition, it can be concluded that compared with other samples,

8

the decrease of surface area on HDTMA modified bentonites was the largest, it may

because bentonite which has the largest CEC may contain more surfactants comparing

with other two clay samples, this conclusion was also confirmed and supported by the

thermogravimetric analysis results as discussed in this paper.

Thermogravimetric analysis (TG): Thermogravimetric analysis (TG) and derivative

thermogravimetric (DTG) of QLD-bentonite, kaolinite, halloysite and surfactant

modified samples were shown in Fig. 3. Several mass-loss steps were observed: The

mass loss before 200 °C was attributed to the dehydration of physically adsorbed

water and water molecules around metal cations such as Na+ and Ca2+ on

exchangeable sites in bentonite (Xi et al., 2004). Dehydration peaks at about 54 °C

and 132 °C from the DTG were observed only for untreated bentonite. The mass

losses occurring from around 200 to 500 °C were observed for H-B-2CEC and H-B-

4CEC. By comparing the DTG results of these organoclays with that of bentonite, it

was concluded that these mass losses were attributed to the decomposition of

surfactants. The mass-loss over the temperature 600 °C was ascribed to the loss of

dehydroxylation of the structural OH units of the bentonite. In addition, similar as

shown in our previous study (Xi et al., 2004; Xi et al., 2007), in the de-surfactant

section (a temperature section where surfactants are decomposed), the DTG peak

numbers of the organoclays were different. For H-B-2CEC, there were two peaks

centred at 263 and 422 °C respectively. For H-B-4CEC, there were three peaks

centred at 231, 302 and 423 °C respectively. According to our previous study (Xi et

al., 2004), when the concentration of the surfactant was relatively low, the organic

cations exchanged with the Na+ ions and mainly adhered to surface sites via

electrostatic interactions. With the increase of the concentration of surfactant, some

surfactant molecules attached to the surface of montmorillonite. This resulted in the

9

appearance of the second peak. If the concentration increased further, the

concentration of the surfactant exceeded the CEC of clay. Surfactant molecules then

adhered to the surface-adsorbed surfactant cations by van der Waals forces. The

properties of these organic cations were very similar to that of pure surfactant. So the

third peak appeared and was very close to the temperature of the pure surfactant. In

addition, with the increase of the concentration of the surfactant, the temperature of

the third peak decreased gradually (became closer to that of the pure surfactant).

Usually, there was a considerable difference between the temperature of pure

surfactants and surfactants in the organoclays. This may be due to some surfactants

attached strongly to the montmorillonite, which caused an increase in the

decomposition temperature (Xi et al., 2004; Xi et al., 2007). As for kaolinite, there

was one broad peak centred at about 502 °C as determined from DTG, and for

HDTMA modified kaolinite, there were two peaks at 260 and 503 °C respectively for

H-K-2CEC and 250/495 °C for H-K-4CEC. The peaks at around 250 to 260 °C were

attributed to the surfactant decomposition. While for halloysite, it was similar to that

of kaolinite which showed only one peak at 492 °C and for H-H-2CEC and H-H-

4CEC, except for the peak from halloysite there were two peaks at 254 and 253 °C

from surfactant decomposition for each of the organoclays respectively. As was

apparent from the XRD patterns and TG curves, the surfactants were successfully

intercalated into these clays’ interlayers or on the surface of these materials. And

according to our pervious study, there may be differences between the amounts of

surfactant added and loaded (Xi et al., 2006). The loaded surfactant amount could be

determined by thermogravimetry (TG). According to Eq. (1) (Xi et al., 2006), the

loaded amount of the surfactants could be calculated, and the results were summarised

in Table 1 - where X is the loaded surfactant amount; m is the total weight of

10

organoclay; M is the molecular weight of surfactant; S% is the weight loss percentage

of surfactant in organoclay; y is 0 (if all the Br ions remain) or 80 (no Br ion, the

molecular weight of Br is 80), y = 0 or 80 is not possible to reach but it can help to

work out the range of X by calculating theoretical maximum and minimum values. As

shown in Table 1, there are differences between the amount of surfactant added and

the amount of surfactant loaded in organoclay. It was evident that for all these

organoclays there were less surfactants loaded in the clays than that predicted by the

theoretical values. For HDTMA modified QLD-bentonite, the data fitted quite well to

the theoretical values for both 2 and 4CEC modified ones. While for HDTMA

modified kaolinite, there were less surfactant loaded than those of added values, and

for HDTMA modified halloysite, it was concluded that only about 0.6-0.9 CEC really

loaded on this clay no matter the initial surfactant added was 2 or 4CEC. This

conclusion also explained the decreases of surface areas on HDTMA modified

bentonites which were the largest as discussed at the beginning, since more

surfactants were loaded on the clay surface and thus blocked some surface area.

Equation 1

Nitrate adsorption on clays and organoclays: This study was carried out by using

100 mg/L nitrate solution to compare the sorption capacity of different organoclays.

The nitrate removal capacity for each sample was shown in Fig. 4. Untreated QLD-

bentonite did not remove any nitrate ions from the solution, and kaolinite showed

similar behaviour for nitrate removal. Halloysite can remove about 0.54 mg nitrate

ions per gram of clay. Generally speaking, all these untreated clays showed poor

adsorption capacities for nitrate ions. While when these clays were modified with

(M-y) · m · (100-S) · 10-2 · 76.4 · 10-3

m · S · 10-2 · 100

76.4 · (M-y) · (100-S)

S · 105

X = =

11

surfactant HDTMA in 2 or 4 CEC, the removal capacities of these materials were

greatly improved as shown in Fig. 4. Among all these organoclays, HDTMA modified

QLD-bentonite showed the best result: H-B-2CEC and H-B-4CEC can remove 12.83

mg and 14.76 mg nitrate ions per gram of organoclay respectively. But HDTMA

modified kaolinite and halloysite removed less nitrates compared to H-B samples, for

example, H-K-2CEC, H-H-2CEC and H-H-4CEC may remove 1.54, 1.78 and 1.93

mg nitrate per gram of clay respectively. When the concentration of HDTMA was

increased to 4CEC, the removal capacity was increased to 4.87 mg/g on H-K-4CEC.

It is probably because that the CEC of QLD-bentonite (66.67 meq/100 g) is higher

than that of kaolinite and halloysite (9.78 and 10 meq/100 g respectively). So when

modified with HDTMA, there were more surfactant molecules exchanged and

attached on the surface of QLD-bentonite than that of other two clays. These

surfactants will help to increase nitrate removal capacities due to changes in the

surface property of the clay hybrids during their modification process where these

surfactant cations can attract and electrostatically hold anionic contaminants. And it

also showed that surface areas of organoclays were not a key factor controlling the

nitrate adsorption.

4. Conclusions

Comparing with original clay minerals, modification with HDTMA decreased

the specific surface areas. The utilization of HRTG allows one to determine the

thermal behaviour of organoclays prepared by surfactants to provide information on

their configuration and structural changes. TG was also proved to be a useful tool to

estimate the amount of surfactant within the organoclays. These untreated clay

minerals are ineffective materials for nitrate. When modified with cationic surfactants,

12

they have potential to be used for anionic contaminants remediation and among them

HDTMA modified bentonites have much better adsorption capacities comparing with

other organoclays investigated.

Acknowledgements

The authors would like to acknowledge the financial and infrastructural

support of the Centre for Environmental Risk Assessment and Remediation (CERAR,

University of South Australia) and the Cooperative Research Centre for

Contamination Assessment and Remediation of the Environment (CRC CARE). And

we are very grateful to the reviewers and editors for their constructive comments.

13

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15

List of Figures

Fig. 1 XRD patterns of QLD-bentonite and relative organoclays

Fig. 2 XRD patterns of kaolinite, halloysite and relative organoclays

Fig. 3 DTG results of clay minerals and organoclays

Fig. 4 Nitrate adsorption on QLD-bentonite, kaolinite, halloysite and organoclays

List of Table

Table 1 Real surfactant loading of organoclays

16

Fig. 1 XRD patterns of QLD-bentonite and relative organoclays

17

Fig. 2 XRD patterns of kaolinite, halloysite and relative organoclays

18

Fig. 3 DTG results of clay minerals and organoclays

Fig. 4 Nitrate adsorption on QLD-bentonite, kaoliniteite, halloysite and organoclays

0

2

4

6

8

10

12

14

16

QLD-bentonite

Kaolinite Halloysite H-B-2CEC H-B-4CEC H-K-2CEC H-K-4CEC H-H-2CEC H-H-4CEC

Nit

rate

rem

oval

(mg/

g)

Nitrate adsorption on organoclays

19

Sample H-B-2CEC H-B-4CEC H-K-2CEC H-K-4CEC H-H-2CEC H-H-4CEC CEC added 2.0 4.0 2.0 4.0 2.0 4.0 Surfactant (%) in organoclay

29.92 43.17 5.36 8.67 2.36 2.31

Real exchanged value (with Br ion)

1.76 3.13 1.59 2.66 0.66 0.65

Real exchanged value (without Br ion)

2.25 4.01 2.58 3.41 0.85 0.83

Table 1 Real surfactant loading of organoclays