Indian Jo llrnal o f C hemi cal Technc lagy Vol. 9, July 2002, pp. 285-28Y

Articles

Iron oxide coated sand as an adsorbent for separation and removal of phenols

OK Singh" , Bhavana Sri vastava & Pushpa Yadav

Depart ment of Chemi stry , Harcourt Butler Technological Institu te , Kanpur 208 002, Indi a

Recei ved II }lI/lIIary 2002; I'(' l 'i.l'ed recei ved 29 April 2002; aC('er>led 13 May 2002

Iron oxides were coated onto the surface of sand, and this composite media was used for adsorption studies of phenolic compounds. These form complexes on the surface of hydrous iron oxide. Probably, the coordination of ligands with the central metal ion occur§ through the phenolic oxygen. Adsorption capacities, rate of adsorption and breakthrough curves were studied. The adsorption of phenols remains same in the pH range 3-6. Distribution coefficients (Kd ) of 17 phenols were determined in water and sodium hydroxide solutions (0.1-0.5 M). On the basis of difference in Kd values some quantitative separation of phenols were achieved. The adsorbed phenols were desorbed quantitatively with 1 M NaOH and the same column could be used for 4-5 cycles consecutively. A small phenol removal unit was also studied for phenol removal from water.

Separation and removal of phenols by adsorption is an emerging fi eld of research . Anion exchange resin l

,

iron(lII ) diethanolamine2, zinc silicate" stanni c

tungstate4, zinc silicate in Fe(lll ) forms, iron(JII )

hydroxide impregnated saw dust6, iron(llI ) hydrox ide loaded marble7

, ni ckel, cobalt and cadmium t· 'd 8 d . d b l) 10 I h errocyal11 es an actl vate car ons ' lave een reported fo r the separation and removal of phenols.

The phenols in water environment can ari se from natural substance degradation, industrial activiti es and agriculture practices. Phenols especially chlorinated phenols may be life-threatening to humans even at I . 1111 Th ' . ow concentratIOns ' '-. elr presence gives di sagreeable smell and taste even at few ppb concentrations 13. The Environmental Protection Agency (US-EPA) includes in Federal Register list of eleven substituted phenols retained hazardous fo r human health and ass igns them a maximum admissible concentration range of 60-400 p, g/L in relation with their tox icity degree l 4

.

The ability of metal ox ide in general and iron ox ide in particular to adsorb both inorganic ions 15 and water born humic type compounds 16-23 is well-known. Iron oxide coated sand (lOCS) has been reported for the adsorption of metal ions24 and natural organic matters25 . The present study reports separation and removal of phenols using IOCS as a new adsorbent.

Experimental Procedure Iron ox ide coated sand was prepared using the

method reported by Benj amin et a1. 24. In the first step,

*For correspondence (E-mail : bhavanakan@rediffma il.com; Fax: 9 1-51 2-29485 1-55 )

80 mL of 2.5 M FeCb solution was poured over 200 mL bulk sand (100-1 20 mesh), stirred and heated for 3 h at SSO± 1°C, The coating was dark coloured almost black. Upon rinsing with di stilled water, the bl ack coloured fraction washed away but a dark red coating remained on the sand. The FeCb solution used to regenerate the coating is strongly acidic, due to hydrolysis of the Fe+3 ions by reactions of the type.

FeCI3 + 3 H20 ~Fe (OH ):, + HCI

During the heating step, both water and HCI evaporate from the solution, neutralizing and concentrating the residual solution and causing iron oxide to prec ipitare. This material will be referred as HTM (high temperature media) in further discussions.

SSO±IOC 2 Fe(OHh ---~ Fe20 3 + 3 H20

In next step, 40 mL of HTM was placed in a heat resistant di sh in a layer 1-3 cm deep, and was mixed with a solution of 80 mL of 2.5 M Fe(N03h (PH- 6). The mixture was covered loosely , heated at llO± 1 °C and stirred until it appeared to be dry (10-12 h). After cooling at room temperature, iron oxide coated sand was sieved through ISO mesh sieve to remove free iron oxide particles .

Determination of iron loaded on IOCS IOCS (1 g) was treated with 20 mL hydrochloric

ac id ( 12 M) and filtered. The iron content in the filtrate was determined spectrophotometrically with I, I 0-phenanthroline26.

Articles

Metal leakage Iron leakage into the solutions under investigation

was determined by shaking 0.5 g IOCS with 50 mL solutions for 4 h at room temperature. Iron dissolved into the equilibrated solution was determined spectrophotometrically 26.

Phenol adsorption procedure Adsorption experiments were performed by

shaking 50 mL adsorbate solution (0.1 gIL) with 0.1 g adsorbent in a stoppered Pyrex glass flask for 6 h at 25± I dc. The Follin's reagent procedure27 was used for spectrophotometric determination of phenols except nitrophenol which was directly estimated at 360 nm.

Kinetic measurements The kinetics of phenol adsorption on IOCS was

studied using batch technique. A number of stoppered conical flasks containing 20 mL of phenol solution (0.1 gIL) and 0.2 g IOCS were mechanically agitated. The samples were collected at specific time intervals for the purpose of sorption kinetics and analyzed .

Breakthrough capacity The breakthrough behaviour for different phenols

was studied by passing their solutions (1 mg/ 1 0 mL) through a glass column (1.2 cm i.d.) loaded with 2 g IOCS. The flow rate was maintained - 0.5 mL / min .

Column separations For quantitative separations of phenols, IOCS (2g)

was taken into a column of i.d. 0.4 cm. The column was washed with 20 mL of distilled water and the mixture of phenols was introduced into the column. The adsorbed phenols were eluted separately using appropriate eluents (Table 2).

Filtration experiment A small filtration unit was fabricated by connecting

a polyvinyl chloride (PYC) tube (40 cm long,S cm i.d .) to a Buchner funnel that had been cut to one cm length . A glass fiber filter with nominal particle retention of 2.5 11m was placed above the porous plate of the Buchner funnel (Fig. 1). The column was filled with 500 g (360 mL) of IOCS up to a height of 19 cm and was washed with about I L of distilled water. Then the column was connected to a Marriott bottle, which maintained a constant hydraulic head of 30 cm. Flow rate of the effluent was maintained -I Llh (empty bed contact time 22 min). The performance of this column was evaluated in removing pyrogallol from ground water (PH 7.2-7.5, hardness 300-320 mg

286

Indian J. Chem. Techno!., July 2002

CaC03/L and conductivity 790-850 mhos/cm) spiked to 100 mg/L.

Results and Discussion The resuits of adsorption studies presented in

Table 1 reveal that for investigated phenols adsorption capacities increase with their pKa values (1.8x I 0-2 to 13 .80x 10-2 mmol!g) except pyrogallol. The retention of phenol on iron-oxide coated sand occurs by virtue of Fe-phenol complex formation on hydrous iron oxide. The adsorption mechanism seems to be ligand exchange reaction between ionized phenol and hydroxy group on iron oxide surface. The process is consistent with the studies of natural organic matter removal by coagulation with iron salts28

-30 where the

dominant sorption mechanism is assumed to be ligand exchange reaction 17.22.23.29.

HTM. FeO.OH + ROH--7HTM. FeO.RO + H20

From mariotte bottle

3 em

I---.-::L----j_ Constant head

-PVCtube

_ Glass fiber filter

-IOCS

<- Buchner funnel

Stop cock

Fig. I- Small filtration unit

Table I- Adsorption of phenols on IOCS

Phenol pK" Adsorption capaci ty x \02

(m mol/g)

p-Nitrophenol 8.20 1.80

2 -Ch lorophenol 8.48 2.33

Resorcinol 9.44 3.45

Pyrocatechol 9.45 7.36

Phenol 9.90 4.67

Ill-Cresol 10.10 4.53

Pyrogallol 12.50 13.80

Singh el al.: Iron oxide coated sand as adsorbent for separation of phenols Articles

The results of chemical analysis of iron oxide coating revealed that iron content on 10CS surface (g Fe/g 10CS) is 0 .074.

The solubility data of immobilized iron given in Table 2 show that iron oxide coating is stable in water, ethanol, 1 M NaOH and acidic solution up to pH 1. The results of kinetic studies reveal that equilibrium is attained within 50 min.

The results of adsorption behaviour of phenols studied at different pH (3-1 (\ \ reveal that adsorption is highest and almost constant in the pH range 3-6. It seems due to stronger interaction between positively charged surface of iron oxide particles and phenols at acidic pH. The lower adsorption capacity observed at pH > 7 may be due to the presence of competing hydroxide ions.

The breakthrough curves for phenols plotted in Fig. 2 reveal that 9 bed volumes of m-cresol, 16 bed volumes of pyrocatechol and 34 bed volumes of

Table 2- Solubility of immobilized iron in different solvents

Solvent

Demineralized water

Ethanol

Sodium hydroxide 0.1 M

Sodium hydroxide 1.0 M

Sodium hydroxide 2.0 M

Hydrochloric Acid 0.1 M

Hydrochloric Acid 0.5 M

o ()

o

1.0

0 .8

0.6

0 .4

0.2

Amount of iron released /lg/L

0.00

0.00

0.00

0.00

1.36

0.60

2.50

pyrogallol, corresponding to a retention of 9, 16 and 34 mg respectively, can be passed through a column of 10CS without any trace being detected in the effluent. On uncoated sand, breakthrough was observed in the first bed volume only .

In order to study the selectivity behaviour of 10CS the distribution coefficient of (Kd values) of 17 phenols were measured in 5 different systems. The results are summarized in Table 3 .

K _ Concentration of phenol in adsorbent phase

d - Concentration of phenol in solution phase

The Kd values reveal that 10CS has differential selecti vity toward phenols. In aqueous media pyrogallol is strongly retained by the adsorbent; m-cresol, o-aminophenol, phenol and pyrocatechol only partially while 0- and p-cresol, m- and p-aminophenol, resorcinol, phloroglucinol, 0.­

naphthol, p-nitrophenol, 2-chlorophenol, picric acid 2,4,6-trichlorophenol are scarcely adsorbed. The data show that Kd values decrease with the increasing concentration of NaOH solution. This decrease in Kd values is due to desorption of phenols by NaOH solution (i.e. , OR ligand).

The analytical utility of the 10CS has been demonstrated by achieving the quantitative separation of phenols (Table 4). The positional isomers such as phloroglucinol-pyrogallol, and o.-naphthol-,B-naphthol were easily separated on the columns of 10CS. Other

Pyrocatechol pyrogall I

200 240 280 320 400 440

Effluent, mL

Fig. 2-Breakthrough curves of phenols on column operation . Co and C denote initial and final concentrations in each effiuent fraction , respectively

287

Articles Indian J. Chem. Techno!., July 2002

Table 3-Kd values of phenols on rocs Phenol Kef values

Ethanol H2O 0.1 M 0.3M 0.5M NaOH NaOH NaOH

Phenol 41.6 47.0 30.8 20.1 15.5

a-Cresol 15.8 17.6 8.4 1.6 0.8

III-Cresol 52.0 54.3 40.2 29.7 20.6

p-Cresol 36.4 38.8 22.4 11.3 8.7

a-Aminophenol 53.4 60.5 58.0 41.2 18.7

m-Aminophenol 28.7 37.6 18.7 7.2 0.0

p-Aminophenol 22.9 29.6 19.8 9.4 5.2

Pyrocatechol 63.2 69.5 68.2 50.0 42 .3

Resorcinol 33.2 37.6 9.6 3.8 0.6

Pyrogallol 240.0 266.9 125.0 98.6 80.2

Phloroglucinol 32.8 40.2 10.8 2.6 1.2

a-Naphthol 20.6 28.1 12.4 5.0 1.8

fJ-Naphthol 0.0 0.0 0.0 0.0 0.0

p-Nitrophenol 20.0 26.3 14.6 6.8 3.6

2-Chlorophen 23.4 31.9 20.4 10.0 6.0

Picric Acid 16.2 17.6 7.8 1.4 0.2

2,4,6-Trichlorophenol 28.6 39.0 26.8 14.2 8.2

Table 4-Separations achieved on roes columns

Mixture Eluent mL

Phloroglucinol O. IM NaOH

Pyrogallol 0.5 M NaOH

Phenol 0.1 M NaOH

Pyrogallol 0.5 M NaOH

fJ-Naphthol H2O

a-Naphthol 0.1 M NaOH

fJ-Naphthol H2O

a-Aminophenol 0.1 M NaOH

fJ-Naphthol H2O

Pyrocatechol 0.5 MNaOH

2-Chlorophenol 0.1 M NaOH

Pyrogallol 0.5 M NaOH

binary separations are phenol-pyrogallol, ,B-naphthol -o-aminophenol, ,B-naphthol-pyrocatechol and 2-chlo­rophenol-pyrogallol. The order of elution and eluents for phloroglucinol-pyrogallol and a-naphthol-,B­naphthol plotted in Fig. 3a and 3b show that only small volumes of eluents were required to give compact chromatograms and tailing was insignificant.

The desorption of adsorbate and regeneration of column is an important process in wastewater treatment. To assess the cyclic utility of the adsorbent,

288

Eluate Amount Amount %

30

50

40

50

30

30

30

40

30

40

30

40

loaded found Error (/lg) (/lg)

300 303.6 +1.2

500 492.0 -1.6

300 303.4 +1.1

500 491.2 -1.8

300 304.0 +1.3

400 393.8 -1.5

300 304.2 +1.4

500 488.4 -2.3

300 303.8 +1.3

400 393.2 -1.7

400 404.8 +1.2

500 491.0 -1.8

a glass column SOx1.2 cm was loaded with 2 g IOCS. The phenol solution (100 mg/L, pH -6) percolated downward at a flow rate - O.S mLlmin till phenol appeared in the effluent (breakthrough point). The adsorbed phenol was desorbed quantitatively by 1 M NaOH solution as the sodium phenolic compounds. The column was washed with distilled water until the washings were neutral and then treated with acidic water (PH - 3) and new cycle began. The observed cyclic breakthrough capacity values for pyrogallol

Singh et al.: Iron oxide coated sand as adsorbent for separation of phenols Articles

~ __ ;0.~1~M~~,--~0.~5~M~Na~O~H--~ 250

200

:; 150

(;

i ~ 100

1 ... 50

20 40 60 80 100

Volume of eluent (mL)

120

Fig. 3a-Separation of phloroglucinol and pyrogallol

~.o.:0,-----__ ~ ___ --~O.l'-'M::...:N.=a.::..:OH"---__> 250

200

a. - Naphthol

60 80 100 120

Volume of clucnl (rnl)

Fig. 3b-Separation of ~-naphthol and a-naphthol

(17.0,16.4,16.0,15.7 and 15.3 mg/g) showed that the same column could be used for 5 cycles with only 10 per cent loss in capacity .

The unit demonstrated for pyrogallol removal from ground water treated 80 L (222.2 bed volumes) water in first cycle. 96 per cent pyrogallol was recovered using I M NaOH (lL). The unit treated 77 L (213.8 bed volumes) water in second cycle. The treated water had no pyrogallol content. The proposed unit ' appeared to be promising for phenol removal from wastewaters.

Conclusions Iron oxide coated sand may be useful as an

adsorptive medium for accumulating phenols from water source. The adsorbed phenols can be desorbed quantitatively with 1 M NaOH solution. Quantitative separations of the positional isomers such as a­naphthol-~-naphthol and phloroglucinol-pyrogallol and other phenols in binary mixtures can be obtained successfully on IOCS column.

Acknowledgement Authors would like to thank Head, Department of

Chemistry , and Director, H. B. Technological Institute, Kanpur, India for providing necessary research facilities .

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