Removal of geosmin and 2-MIB in the presence of humic acid using cyclodextrin polyurethanes

Mamba BB, * Krause RW, Malefetse TJ, Sithole SP and Nkambule TI

Department of Chemical Technology, University of Johannesburg, P.O. Box 17011,

Johannesburg, South Africa

*Email: bmamba@uj.ac.za, Telephone number: 011 559 6516, Fax no: 011 559 6425

Abstract The effect of the presence of humic acid on the absorption of geosmin and

2-MIB by water-insoluble on the cyclodextrin polymers was investigated.

Results obtained from this study indicate that the presence of humic acids at

different concentrations could not affect the removal of geosmin and 2-MIB

when cyclodextrin polymers were used. This is in contrast to observations that

were previously reported when activated carbon was used for the same

application, where humic acid was found to compete for adsorption sites with

geosmin and 2-MIB. Herein, we report on the selective removal of geosmin

and 2-MIB using cyclodextrin polyurethanes despite the presence of humic

acids. GC/MS spectrometry and ultraviolet visible (UV) spectroscopic analysis

validated the efficiency of these polyurethanes at removing these organic

species in water.

Keywords: cyclodextrin polymers, geosmin, 2-methylisorboneol (2-MIB),

humic acids

Introduction

Geosmin and 2-methylisorboneol (2-MIB) are organic compounds that cause

bad taste and odour to drinking water. The removal of these compounds from

drinking water has been a challenge for many water authorities. Geosmin and

2-MIB are detectable by the human nose even at 10 ng/L levels. Therefore

highly effective treatment methods are required to remove these compounds

to low concentration levels (Hepplewhite et al, 2004).

1

Natural organic matter (NOM) occurs in all natural water sources when animal

and plant material break down. The main component of NOM is attributed to

humic substances. Humic substances can be divided into three groups

depending on the method of isolation from the original substance. These are

fulvic acid, humic acid (HA) and humin. Humic substances adversely affect

the quality of drinking water since they impart colour and serve as precursors

for the formation of chlorinated compounds. They also have complexing

properties that include association with toxic elements and micro-pollutants.

Activated carbon has often been used in the adsorption of geosmin and 2-

MIB. It has however been noted that the removal of micro organic compounds

including geosmin and 2-MIB, particularly at low concentrations, by activated

carbon is not excellent. This reduced performance can be attributed to the

presence of NOM which is usually found in drinking water at concentration

levels between 2 and 15 mg/L (Hepplewhite et al, 2000). NOM commonly

exists in water at more elevated concentration levels (mg/L) than geosmin and

2-MIB (ng/L). Therefore, NOM tends to compete with geosmin and 2-MIB for

adsorption sites on activated carbon (Tennat, 2004). This consequently

renders the activated carbon ineffective in the removal of the odorous

compounds. NOM, in other words, compromises the adsorption efficiency of

activated carbon significantly resulting in high costs in water treatment

(Newcombe et al, 2002). Another drawback with activated carbon is that it

loses its absorption effectiveness once saturated with moisture and this

causes the adsorbed pollutants to leach out.

In our laboratories the application of insoluble cyclodextrin polymers in water

treatment has been extensively investigated. These polymers have been

tested in the removal of a range of organic compounds present in water at

ng/L and were found to be very effective. (Salipira et al, 2006, Mhlanga et al,

2007 , Mamba et al 2007).

Cyclodextrins (CDs) are cyclic glucose oligomers made from enzymatic

degradation of starch through the action of Bacillus macerans. These glucose

units are joined via α-(1,4) glycosidic linkages (Bender and Komiyama, 1978).

2

The three most commonly known CDs, (α), (β) and (γ), consist of cyclic 6, 7

and 8 membered glucose units, respectively. One outstanding physical

feature of a cyclodextrin molecule is its central cavity which provides an

excellent site for hydrophobic molecules such as organic compounds. The

cyclodextrin cavity is made up of glucose units that are linked by glycosidic

oxygen atoms whilst the outer surface is lined with primary hydroxyl groups in

one rim of the cone and secondary hydroxyl groups in the other rim (Sabini et

al., 2006). The inner cavity is non-polar relative to the outer surface. This

cavity provides a microenvironment into which appropriately sized non-polar

compounds can enter and form inclusion complexes with the cyclodextrin

moiety as depicted in Figure 1.

****************************************************************

Figure 1: Schematic diagram of an inclusion complex formation.

The removal of organic pollutants from water occurs via the formation of

inclusion complexes. The cyclodextrin molecule (host) encapsulates the

organic molecule (guest) resulting in the formation of a complex. However

CDs are highly soluble in water hence limiting their application in the removal

of organic pollutants from water. In order to make them insoluble they are

converted to highly crosslinked polymers by polymerizing the cyclodextrin

monomer with suitable bifunctional linkers such as hexamethylene

diisocyanate (HMDI) (Figure 2) and toluene diisocyanate (TDI) (Li and Ma,

1999)

***********************************************************

Figure 2: Synthetic pathway for the formation of a cyclodextrin polyurethane Therefore, the aim of this study was to synthesise and apply

water-insoluble cyclodextrin polyurethanes for the removal of humic acid,

geosmin and 2-MIB from synthetically prepared water samples. In particular

the objective was to determine if humic acid could have an effect on the

absorption of geosmin and 2-MIB by the cyclodextrin polymers.

3

Experimental

Extraction technique

Solid phase microextraction (SPME) technique was used for the extraction of

geosmin and 2-MIB present in the water samples. The device for this

technique consists of a polymer coated fibre that is fused within a syringe. The

type of polymer coating is chosen such that it matches the characteristics of

the analytes of interest (Nakamura and Daishima, 2005). In this procedure, a

fibre coated with polymethylsiloxane/divynylbenzene (PDMS/DVB) with film

thickness of 65 µm was used (Lin et al., 2003).

To ensure that the fibre was clean before performing an analysis it was first

conditioned. The conditioning was performed by exposing the fibre to injector

analysis temperature of 250°C for 30 minutes. This was followed by a blank

analysis carried out by desorbing the fibre for a further 5 minutes at similar

analysis temperature. Water samples (before and after treatment with

cyclodextrin polymers) were extracted by immersing the fibre in 10 mL

aliquots of the water samples for 30 minutes at 60°C. It was necessary to

constantly stir the sample at the required temperature in order to enhance the

extraction efficiency.

Analysis of water samples using Gas Chromatography/Mass Spectrometry

(GC-MS)

After extracting the organic compounds the SPME fibre was retracted from the

sample and placed directly in the GC injector port to desorb for 3 minutes. The

GC-MS instrument conditions that were used are indicated in Table 1. GC-MS

analysis was carried out using a Varian CP-3800 capillary Gas

Chromatograph coupled with a Saturn 2000 Mass Spectrometer. This

technique was employed because of its ability to separate and uniquely

identify organic compounds from a complex mixture.

4

Table 1: GC-MS method for the analysis of geosmin and 2-MIB extracted by SPME ****************************************************************************************

Preparation of geosmin and 2-MIB standards

Geosmin and 2-MIB standard samples were purchased from suppliers and

standards of 10 ng/L, 50 ng/L and 100 ng/L were prepared since such

concentrations are typical in surface water. SPME was used to extract

geosmin and 2-MIB from the standards and GC-MS was utilised for the

analysis of these compounds. A calibration curve was plotted in order to

determine the concentration of geosmin and 2-MIB after passing the sample

through the polymer.

Preparation of humic acid standard

Humic acid standards were prepared at 1 mg/L, 2 mg/L, 3 mg/L, 5 mg/L,

10 mg/L and 15 mg/L levels using HPLC grade water. The UV absorbance

(at 254 nm) measurements were carried out on a CARY-50 UV

spectrophotometer. A calibration curve was plotted from which the

concentration of the samples was calculated.

Preparation of humic acid, geosmin and 2-MIB multistandard

Three different multistandard solutions of humic acid, geosmin and 2-MIB

were prepared by varying the humic acid concentration while the geosmin and

2-MIB concentration was kept constant. The levels were at 5 mg/L humic acid

and 100 ng/L for geosmin and 2-MIB, 10 mg/L humic acid and 100 ng/L for

geosmin and 2-MIB, 15 mg/L humic acid and 100 ng/L for geosmin and

2-MIB.

Treatment of water samples

The synthetic standard solutions (synthetic water samples spiked with humic

acid, geosmin and 2-MIB) were treated with β-CD/HMDI (a polymer produced

5

from the reaction of β-cyclodextrin and hexamethylene diisocyanate) polymers

(Li and Ma, 1999). The adsorbents (300 mg) were loaded into empty

cartridges and 30 mL of the synthetic water sample was passed through the

polymers at a filtration rate of 10 mL/min. Filtration was further enhanced by

the use of a vacuum pump or water aspirator. The polymer treated water

samples were then extracted using SPME before injection into the GC injector

port. The filtrate was analyzed using the GC/MS to determine the

concentration of geosmin and 2-MIB portions in the treated synthetic water

sample

UV- VIS spectroscopy measurements

The synthetic water samples were also analysed on the UV to determine the

concentration of the humic acid after passing the water sample through the

polymer. The measurement was done at a UV absorbance wavelength of 254

nm.

Results and Discussion Evaluation of the SPME technique Initially solid phase extraction (SPE) and liquid-liquid extraction (LLE)

techniques were used for the extraction of organic compounds. These

techniques proved to be inefficient in the extraction of analytes present in the

water at low concentrations, since GC/MS chromatograms demonstrated very

few signals of the compounds extracted. Hence SPME was adapted and used

for the analysis and this proved to be more successful than SPE and LLE.

The SPME technique is simple to use, economical and quite fast. Additionally

it requires no organic solvents, no dilution of the sample and it can be easily

automated (Guillot et al, 2006). This technique is very efficient in extracting

trace levels of organic compounds such as geosmin and 2-MIB from water

(Waston et al 2000).

Three different fibres namely PDMS (100-µm), PDMS/DVB (65-µm) and CAR

(carboxen)/PDMS (75-µm) were evaluated in order to determine which fibre

6

extracted most of the geosmin and 2-MIB in the water. PDMS/DVB proved to

be the most efficient as a result was adopted for the study. These results are

in line with an experiment conducted by Nakamura and Daishima (2005)

where the PDMS/DVB fibre was used in the extraction of geosmin and 2-MIB.

The fibre gave good extraction yields and the compounds were detected at

very low concentrations (ngL-1 levels).

GC/MS analysis

The effect of the presence of humic acid, on the absorption of geosmin and 2-

MIB was studied using synthetic water samples spiked with humic acid,

geosmin and 2-MIB which were treated with CD polymers. Figure 3 - 5 shows

GC/MS chromatograms of the spiked water samples before and after

treatment with β-CD/HMDI which had been prepared in quantitative yields. It

should be noted that GC/MS was used solely for the analysis of geosmin and

2-MIB and not humic acid. Geosmin and 2-MIB peaks were identifiable at

retention time of 13.6 minutes and 10.2 minutes respectively (Mamba et al,

2007). Treatment of the water samples spiked with humic acid, geosmin and

2-MIB showed that the polymers absorbed up to 90% (despite the increase in

the concentration of humic acid) (Figure 3B, 4B and 5B) of the geosmin and

2-MIB when comparing the ion counts before and after passing through the

polymer.

********************************************************* Figure 3: GC/MS chromatogram of 5mg/L humic acid and 100 ng/L geosmin and 2-MIB before (A) and after passing through β-CD/HMDI (B). ********************************************************************

Figure 4: GC/MS chromatogram of 10mg/L humic acid and 100 ng/L geosmin and 2-MIB before (A) and after passing through β-CD/HMDI (B). ****************************************************************** Figure 5: GC/MS chromatogram of 15mg/L humic acid and 100 ng/L geosmin and 2-MIB before (A) and after passing through β-CD/HMDI (B).

The high percentage absorption of geosmin and 2-MIB onto the polymers,

demonstrates that humic acid did not compete for absorption sites on the

7

polymers. However, Newcombe et al (2002) found that site competition

seemed to be the main factor affecting the adsorption of geosmin and 2-MIB

by activated carbon in the presence of NOM. In that study it was found that,

smaller NOM compounds were mostly adsorbed as they participated in direct

competition with 2-MIB for adsorption sites on activated carbon. This was

attributed to the fact that small NOM compounds are of low molecular weight

(< 600 g/mol) and posses molecular structures that are similar to 2-MIB. In

another investigation (Hepplewhite et al 2004), it was confirmed that

competition for adsorption sites on activated carbon was between low

molecular weight NOM compounds and 2-MIB.

In our study, since the GC/MS analysis showed that geosmin and 2-MIB are

efficiently absorbed by the polymers without any interference from the humic

acid, it was imperative that UV analysis be conducted before and after

passage of water samples through the polymers in order to ascertain whether

the humic acids had been absorbed by the polymer. UV analysis would

hopefully quantitatively give an indication of how much, if any, of the humic

acids had been retained by the polymer.

UV-VIS spectroscopy analysis

UV-VIS analyses in Table 2 show that the polymers did not remove humic

acid ahead of geosmin and 2-MIB. Humic acid concentration levels before and

after treatment with the cyclodextrin polymers remained unchanged. This

suggests that the odorous compounds being smaller in size, compared to

humic acids were most efficiently removed by the CD polymer - the diameter

of the CD ring probably accounting for such selectivity. It has been suggested

by Newcombe et al (2004) that 2-MIB could be expected to reach the

absorption sites before pore blockage by larger compounds such as humic

acids.

Noteworthy, is that the obtained results in this research further confirm our

hypothesis that the water-insoluble CD polymers are most probably suited for

the removal of micro-organic pollutants from water than bigger organic

compounds such as humic acids. While activated carbon would still remove

8

humic acids during water treatment, CD polymers would ideally be positioned

after activated carbon in the water treatment train. The prospect of using the

cyclodextrin polyurethane in a pilot water treatment plant is currently being

pursued.

9

Table 2: Percentage absorption of humic acid after passing though β-CD/HMDI *************************************************************************************

Conclusion Geosmin and 2-MIB were absorbed efficiently by the cyclodextrin

polyurethane polymers despite the presence of humic acids. This could be

attributed to the difference in the sizes of the organic compounds- humic acids

being larger than geosmin and 2-MIB. Practically, the findings in this study

suggest that these polymers would be suited for the penultimate stages of the

water treatment train after ozonation, UV and activated carbon have been

used for the removal of NOM. The CD polymers, in the latter stages, would

then absorbed micro-organic pollutants that were not removed in the earlier

stages of water treatment.

Acknowledgements The authors wish to acknowledge with gratitude funding for this project

obtained from National Research Foundation (NRF), University of

Johannesburg and Eskom’s Tertiary Support Programme (TESP).

References 1. Bender ML and Komiyama M., 1978. Cyclodextrin chemistry, Springler-

Verlag Berlin Heidelberg, NewYork. 1-3

2. Cook D, Newcombe G, and Szatajnbok P., 2001. The application of

powdered activated carbon for MIB and geosmin removal: predicting PAC

doses in four raw waters. Water Research 35, 1325-1333.

3. Guillot S, Kelly MT, Fenet H and Larroque M., 2006. Evaluation of solid-

phase microextraction as an alternative to the official method for the analysis

of organic micro-pollutants in drinking water. Journal of chromatography A

1101, 46-52

10

4. Hepplewhite C, Morrison.J, Newcombe G and Knappe D.R.U., 2002.

Simultaneous adsorption of MIB and NOM onto activated carbon.II.

Competitive effects. Carbon 40, 2147-2156.

5. Hepplewhite C., Newcombe G and Knappe D.R.U., 2004. NOM and MIB,

who wins in the competition for activated carbon adsorption sites? Water

Science and Technology 49, 257–267.

6. Li D and Ma M., 1999. Nanosponges: from inclusion chemistry to water

purifying technology Chemtech 5, 31

7. Lin T, Liu C, Yang F and Hung H., 2003. Effect of residual chlorine on the

analysis of geosmin, 2-MIB and MTBE in drinking water using the SPME

technique Water Research 37, 21-26.

8. Mamba B.B., Krause R.W., Malefetse R.W., Mhlanga S.D., Sithole S.P.,

Salipira K.L., Nxumalo E,N. 2007. Removal of geosmin and 2-

methylisorboneol (2-MIB) in water from Zuikerbosch Treatment Plant (Rand

Water) using β-cyclodextrin polyurethanes. Water SA 33, 223-227.

9. Mhlanga SD, Mamba BB, Krause RW and Malefetse TJ., 2007. Removal of

organic contaminats from water using nanosponges cyclodextrin

polyurethanes. J Chem Technol and Biotechnol 82, 382-388.

10. Nakamura S and Daishima S., 2005. Simultaneous detection of 22 volatile

organic compounds, methyl-tertbutyl ether, 1,4- dioxane, 2-methylisorboneol

and geosmin in water by headspace solid phase microextraction-gas

chromatography-mass spectrometry. Analytica chimica Acta 548, 79-85.

11.Sabadini E, Cosgrove T, and Egio F., 2006. Solubility of

cyclomaltooligosaccharides (cyclodextrins) in H2O and D2O: a comparative

study. Carbohydrate Research 341, 270-274.

12. Salipira K.L., Mamba B.B., Krause R.W., Malefetse T.J., Durbach S.H.,

2007. Carbon nanotubes and cyclodextrin polymers for removing organic

pollutants from water. Environ Chem Lett 5, 13-17.

13. Tennant M.F., 2004. Activation and use of powdered activated carbon for

removing 2-methylisorboneol in water utilities. PhD. Thesis, The Graduate

School of the University of Florida, Florida, United States of America.

11

14. Waston SB, Brownlee B, Satchwill T and Hargesheimer EE., 2000.

Quantitative analysis of trace levels of geosmin and MIB in source and

drinking water using headspace SPME. Water Research 34, 2818-2828.

Tables Table 1: GC-MS method for the analysis of geosmin and 2-MIB extracted by SPME Parameter Condition Column type VF,5ms,30mx0.25mm,0.25µm Injector Splitless, 3 min sample exposure Injector temperature 250°C Oven temperature 50°C (hold for 10 min) to 180°C @ 10°C/min 275°C

@ 40°C/min Ionisation Electron impact Carrier gas Helium Flow rate 1 ml/min Detector Mass spectrometer (ion trap) Mode Full Scan (m/z = 40 - 650) Table 2: Percentage absorption of humic acid after passing though β-CD/HMDI

Sample type

Initial concentration of

humic acid (mg/L)

Final concentration of

humic acid (mg/L)

% absorbed

5 mg/L humic

acid + 100 ng/L

geosmin and 2-

MIB

5 5 0

10 mg/L humic

acid + 100 ng/L

geosmin and 2-

MIB

10 9.43 6

15 mg/L humic

acid+ 100 ng/L

geosmin and 2-

MIB

15 12.33 12

12

Figures

Benzene Cyclodextrin Inclusion complex(Guest) (Host)

Figure 1: Schematic diagram of an inclusion complex formation.

OHOH

OH

hexamethylene diisocyanate

DMF 750C, 24 hrs

OHN

CNC

O

OO

O

H

H

NCN

CO

OO

O

H

HCD CD polymer

Figure 2: Synthetic pathway for the formation of a cyclodextrin polyurethane

13

Geosmin

Geosmin

2-MIB

2-MIB

A

B

Figure 3: GC/MS chromatogram of 5mg/L humic acid and 100 ng/L geosmin and 2-MIB before (A) and after passing through β-CD/HMDI (B).

14

Geosmin 2-MIB

2-MIB Geosmin

A

B

Figure 4: GC/MS chromatogram of 10mg/L humic acid and 100 ng/L geosmin and 2-MIB before (A) and after passing through β-CD/HMDI (B).

15

Geosmin

2-MIB

2-MIB

Geosmin

B

A

Figure 5: GC/MS chromatogram of 15mg/L humic acid and 100 ng/L geosmin and 2-MIB before (A) and after passing through β-CD/HMDI (B).

16