tannin-aniline-formaldehyde resole resins for...

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Borderless Science Publishing 450

Canadian Chemical Transactions Year 2014 | Volume 2 | Issue 4 | Page 450-466

ISSN 2291-6458 (Print), ISSN 2291-6466 (Online)

Research Article DOI:10.13179/canchemtrans.2014.02.04.0123

Tannin-Aniline-Formaldehyde Resole Resins for Arsenic

Removal from Contaminated Water

Khudbudin Mulani, Siona Daniels, Kishor Rajdeo, Sanjeev Tambe and Nayaku Chavan*

Polymer Science and Engineering Division, National Chemical Laboratory, Dr. Homi Bhabha Road,

Pune – 411008, India

*Corresponding Author, E-mail: [email protected] Phone and Fax: 91-20-25903008

Received: April 21, 2014 Revised: June 17, 2014 Accepted: June 18, 2014 Published: June 18, 2014

Abstract: Tannin is the polyphenol reached substrate that has high affinity to bound metal ions. Tannin-

formaldehyde resins were synthesized in the presence of aniline as a comonomer through a covalent bond

and were studied to determine its efficiency for arsenic binding. The adsorption of metal ion on the resins

was monitored by FTIR and EDX analysis. The metal ion adsorption parameters such as contact time and

pH were studied to see effect on binding capacity. The metal ion adsorption capacity was measured by

proton exchange method. The maximum adsorption capacity observed was around 90% for As(V) in the

pH range of 4 - 8 and for As(III) in the pH range of 4 - 5. SEM study of resins was carried out to see the

ligand formation with metal. The experimental results reveal that metal ions bind strongly to tannin

resins. Adsorption/desorption behavior of metal ion on resins was studied for its reusability viewpoint.

Nearly 95% of the adsorbed As(III) and As(V) ions were desorbed from tannin resins by using 1M nitric

acid solution in the first step, which then decreased to 85% at the third step.

Keywords: Tannin-formaldehyde resins, Aniline, Adsorption, Desorption, Arsenic, Metal

recovery/removal.

1. INTRODUCTION

The presence of toxic metals in ground water sources is a serious problem in water and

wastewater management. Arsenic is a ubiquitous element widely distributed in the earth’s crust. It is

released in drinking water supply by either natural sources or industrial processes [1]. Arsenic can cause

serious health issues such as cancers of the skin, lungs and bladder. Epidemiological evidence indicates

that arsenic concentration in drinking water exceeding 50 µgL-1

is detrimental to public health. Toxicity of

arsenic varies greatly according to its oxidation state. Naturally occurring inorganic arsenic generally

exists in two oxidation states, arsenite [(As(III)] and arsenate [(As(V)]. Arsenic commonly present in

water is a pH dependent species of arsenic (H3AsO4) and arsenate (H3AsO3) acid systems. In natural

water and drinking water, it is mostly found as As(III) and As(V) [2,3]. As(III) is sixty times more toxic

than As(V) [4, 5]. The maximum permissible concentration of arsenic, according to World Health

mailto:[email protected]

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Organization (WHO), United States Environmental Protection Agency (US EPA), in drinking water is 10

µg/L [6]. However, arsenic concentrations, about hundred times more than the permissible limit. Several

treatment methods have been investigated for arsenic contaminated drinking water and the main

mechanism used for removal of arsenic from ground water are solvent extraction, chemical precipitation

etc [7, 8]. Other techniques include reverse osmosis, adsorption, ion exchange methods and permeable

reactive barriers [9, 10]. All methods and techniques are associated with disadvantages such as efficiency

of metal removal, high cost of reagents and energy requirement [5-10]. Adsorption has emerged as an

alternate to these traditional methods due to its simplicity and safety, ease of operation, maintenance and

handling, sludge free operation, potential for regeneration and possibility of the use of low cost adsorbent.

So far, various adsorbents for arsenic removal have been developed that include materials such as

activated carbon, activated alumina, activated carbon impregnated with ferric hydroxide and tartaric acid,

metal-loaded coral lime stone [11-14]. Most of these adsorbents however entail several problems in terms

of efficiency and cost.

To improve adsorption capacity and to minimize the drawbacks, several organic and inorganic

supporters have been applied such as cement, sand, slag, resin, cellulose bead and chitosan [15-21]. Anion

exchange fibers were prepared from vinyl benzene chloride precursor for arsenate removal, but this

adsorbent is costly and economically not viable [22]. Recently, Tripathy and coworkers reported granular

activated alumina as a very effective adsorbent for removal of arsenic from aqueous solution [23].

However, it works only at lower pH range (5 - 6.5) and it has more affinity for competing ions such as

fluoride, phosphate than arsenic. Activated alumina was used as adsorbent for the removal of arsenic, but

is less effective adsorbent for As(III), and its efficiency decreases as pH increases [24]. Ion exchange

resin was used as adsorbent but it is costly and economically not viable [25]. Zeolites were used as

adsorbent for removal of arsenic but the adsorption capacity decreases with increases in pH [26].

However, above systems suffered from multiple preparation steps and time consuming processes.

The two main things considered while selecting adsorbent are cost of adsorbent and easy

availability. Basically, low cost adsorbents used are originated from natural sources i.e. natural materials.

Because of low cost and high availability of these natural materials, they receive much attention for use as

adsorbent. Tannins (TA) are one of them and can be widely used as effective adsorbing agent for water

treatment in developing countries. Tannin contains polyphenolic compounds which are more attractive

towards heavy metal ions. Tannins extracted from the bark of the black wattle tree and available

commercially containing multiple hydroxyl groups, exhibiting specific affinity to metal ions can probably

be used as an alternative, effective and efficient adsorbent for the recovery of metal ions [27]. As tannins

are water soluble compounds, when used directly, they are leached by water. To overcome this drawback,

many researchers have been reported to immobilize tannins on to various matrices in order to solve the

water-soluble problem [28]. In the immobilization reaction, the main role of formaldehyde is to react with

phenolic moiety present in tannin and make a polymer which is insoluble in acidic as well as basic media.

As a result, this tannin based adsorbent shows excellent adsorption capacity towards metal ions.

In the literature, adsorbents prepared from commercial condensed tannins are applied for the

removal of heavy metal ions such as uranium, americium, copper, cadmium, chromium, vanadium and

lead [29-34], but the studies of adsorption kinetics for the removal of arsenic metal ions from the effluents

using tannin based sorbent have not been reported.

The main objective of the study was to develop the polymeric material synthesized from natural

source of tannin for the removal of arsenic from waste water. In this paper we address the synthesis,

characterization and arsenic ion adsorption kinetic studies using tannin-formaldehyde resins and tannin-

aniline-formaldehyde resins.

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2. EXPERIMENTAL

2.1 Materials

Tannin, sodium arsenate and sodium arsenite were procured from Loba chemie, Mumbai, India.

Aniline and 25% ammonia solution were purchased from Merck, India. Formaldehyde (37% solution)

was obtained from Qualigens, India. All chemicals were used without purification and distillation.

2.2 Preparation of Tannin-Formaldehyde Resins (TFA resins)

Into a plastic container, 4 g of commercial tannin powder and 10 mL of 37% formaldehyde

solution were added and stirred for five minutes. To the resulting solution, 20 mL of 25% ammonia

solution was added, and continued the stirring for five min. A brown precipitate obtained was kept for

fifteen days, yellow precipitate was observed at the bottom. The reaction mixture was then neutralized

with 10.8 N hydrochloric acid solution and filtered through 0.45 µm Millipore filter paper. The precipitate

obtained was washed with deionized water followed by drying under reduced pressure at 80oC for 10 h.

The monomer composition of tannin-formaldehyde resins using ammonia as catalyst is presented in Table

1.

Table 1. Monomer compositions of tannin-formaldehyde (TFA) resins using ammonia as catalyst.

Resin No.

TA

(g)

FO

(mL)

AM

(mL)

Basic

medium

Acidic

medium

TFA 01 4.0 50 40 Tm S

TFA 02 4.0 40 40 Tm S

TFA 03 4.0 30 40 Tm S

TFA 04 4.0 20 40 Tm S

TFA 05 4.0 10 40 Tm S

TFA 06 4.0 50 20 S L

TFA 07 4.0 40 20 S L

TFA 08 4.0 30 20 Tm S

TFA 09 4.0 20 20 Tm S

TFA 10 4.0 10 20 Tm L

TA – Tannin; FO – Formaldehyde; AM – Ammonia; S - Solid powder;

Tm - Thick mass; L - Liquid.

2.3 Preparation of Tannin-Aniline-Formaldehyde Resin (TAFA resins)

Into a plastic container 3 g of commercial tannin powder, 1 mL of aniline, and 10 mL of 37%

formaldehyde solution were placed and stirred for five min. To the resulting solution, 20 mL of 25%

ammonia solution was added and stirring was continued for additional five min, and a brown precipitate

obtained was kept for fifteen days. The resulting mixture was neutralized by 10.8 N hydrochloric acid

solution, and yellow precipitate obtained was separated by filtration and washed with deionised water

followed by drying under reduced pressure at 80oC for 10 h. The monomer composition of tannin-aniline-

formaldehyde resins using ammonia as catalyst is given in Table 2.

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Canadian Chemical Transactions Year 2014 | Volume 2 | Issue 4 | 50-466


Table 2. Monomer compositions of tannin-aniline-formaldehyde (TAFA) resins using ammonia as

catalyst.

Resin No.

TA

g

AN

mL

FO

mL

AM

mL

Basic

medium

Acidic

medium

TAFA 01 3.0 1.0 50 40 L S

TAFA 02 3.0 1.0 40 40 L S

TAFA 03 3.0 1.0 30 40 L S

TAFA 04 3.0 1.0 20 40 L S

TAFA 05 3.0 1.0 10 40 L S

TAFA 06 3.0 1.0 50 20 Tm S

TAFA 07 3.0 1.0 40 20 Tm S

TAFA 08 3.0 1.0 30 20 Tm S

TAFA 09 3.0 1.0 20 20 Tm S

TAFA 10 3.0 1.0 10 20 Tm S

TA- Tannin; AN-Aniline; FO – Formaldehyde; AM – Ammonia; S - Solid powder; Tm - Thick mass;

L - Liquid.

2.4 Adsorption Studies

The arsenic solutions were prepared by dissolving 0.173 g of sodium arsenite (NaAsO2) in 1000

mL deionised water for As(III) and 0.450 g of sodium arsenate (NaH2AsO4.7H2O) in 1000 mL deionised

water for As(V) respectively.

Adsorption experiments were carried out by agitating 1 L of metal ion solution with 1 g of tannin

resins at room temperature. When the initial pH of the adsorption medium was adjusted to a higher value

of pH 4, some of the metal ions were precipitated due to existence of OH- ions in the adsorption medium.

The mixture was stirred continuously at 150 rpm for 3 h, the solutions were filtered through 0.45 µm

Millipore filter paper. The amounts of metal adsorbed were calculated from the concentration in solutions

before and after adsorption process.

Batch sorption experiments were conducted to examine the adsorption of As(III) and As(V) ions

on TFA and TAFA resins. Adsorption isotherms were obtained for individual metal ions under similar

conditions. The sorption was also measured as a function of time and pH under similar conditions.

2.5 Measurements Infrared spectra of resins were obtained with a FTIR spectrophotometer. The resin was dried at

80oC under reduced pressure for 8 h where as potassium bromide (spectrometry grade) was dried at 300

oC

for 4 h. Potassium bromide pellet was prepared by mixing 1 mg of resin with 100 mg of KBr at 10000

kg/cm2 pressure for 30 min under vacuum. The spectra were recorded using spectral width of 4000-450

cm-1

(8 scans). The particle size of the tannin resins was determined by AccuSizer 780 Optical Particle

Sizer, Santa Barbara, USA and particle size distribution of the resin is reported in Table 3. The results

indicated that the particle size was in the range of 0.8 - 2.0 μm and narrower particle size distribution of

the particles compared to suspension polymerization.

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Table 3. Data of particle size and exchangeable protons of tannin-formaldehyde (TFA) resins.

Sample code

Particle size

(μm)

H+

mmol/g

Sample code

Particle size

(μm)

H+

mmol/g

TAFA 01 0.81 8.3 TFA 01 1.48 4.9

TAFA 03 0.78 8.2 TFA 02 0.93 5.8

TAFA 04 1.51 8.7 TFA 03 1.71 7.6

TAFA 05 0.99 8.0 TFA 04 1.46 7.9

TAFA 06 0.87 8.1 TFA 05 0.98 8.1

TAFA 07 0.81 9.6 TFA 08 1.23 7.6

TAFA 08 0.80 9.4 TFA 09 1.27 7.9

TAFA 09 0.83 8.9 TFA 10 0.88 8.1

TAFA 10 0.85 7.4

2.6 Analytical Methods

The residual arsenic in water sample was determined using molybdenum blue method [35]. The

method was used to estimate As(III) and As(V) concentrations in treated water samples to assess the

efficiency of the oxidation step and subsequent removal of arsenic. Spectrophotometric measurements

were made at a wavelength of 865 nm using absorbance cell of 1 cm path length for arsenic

determination.

Procedure: Two sets of concentrations starting from 1-15 ppm/L of As(III) to As(V) were

prepared from the standard solutions. One set is used for treated (oxidized) aliquot and other for untreated

one. One mL of 1 N hydrochloric acid and 1 mL of 0.017 M potassium iodate were added successively to

each of the treated aliquots with thorough mixing after each addition. Ten minutes were allowed for

oxidation of As(III) to As(V), and 4 mL of mixed reagent was added to each of the treated (oxidized) and

untreated aliquots with thorough mixing. After 2 h the complex was formed and developed a blue color.

The amount of complex formation is directly proportional to the arsenic concentration, which was

determined as a function of absorbance measured at 865 nm wave lengths with an UV spectrophotometer.

Suitable blank samples were run using above procedure along with the samples for two times.

Mixed reagent preparation: The solutions of 15 mL of 0.1 M ascorbic acid (freshly prepared), 7.5

mL of 0.032 M ammonium molybdate, and 2.5 mL of 0.0082 M potassium antimony tartarate were

mixed. To this solution, a solution of 25 mL of 5N sulfuric acid was added immediately after the addition

of potassium antimony tartarate to avoid the generation of turbidity in the color reagent.

2.7 Determination of Exchange Capacity of the Resins

The exchange capacity of the tannin resin was measured by the proton exchange method [36].

The

proton titration experiments were carried out to determine the number of valid sites for adsorption present

on adsorbent particles. Into a flask, 10 mL of 0.1 N sodium hydroxide solution and 0.1 g of resin particles

were placed and the flask was kept in a thermostatic water shaker bath and shaking was continued for 24

h at 25oC at 100 rpm. The stirred solution was titrated against 0.1 N hydrochloric acid solution using

phenolphthalein as an indicator.

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3. RESULTS AND DISCUSSION

3.1 Tannin-Aniline-Formaldehyde Resins

Tannins are composed of polyphenolic compounds having higher molecular weights ranging from

500-20000 g/mol. Due to the enormous structural diversity, tannins are divided into two major groups: (a)

condensed and (b) hydrolysable tannins. Generally, low molecular weight tannins are soluble in water.

Condensed tannins are oligomeric and polymeric proanthocynidins formed by linkage of C-4 on one

catechin with C-8 or C-6 of the next monomeric catechin [37]. A-ring of the condensed tannin serves as

very reactive nucleophiles, where as B-ring provides antioxidant properties and excellent sites for

complexation with metal ions due to the presence of ortho-hydroxyl groups [38, 39]. The nucleophile

centers on the A-ring of flavanoid unit tend to be more reactive than those on the B-ring. This is due to the

presence of vicinal hydroxyl substituent and also steric hindrance. Therefore formaldehyde reacts with

tannin to produce resin through methylene bridges at reactive positions on the flavanoid molecules of the

A-ring. The free C-6 and C-8 sites on the A-ring react with formaldehyde due to the strong nucleophilicity

to form insoluble tannin resin. The high reactivity of tannin towards formaldehyde is the result of the

reactivity of A-ring which has 10 to 50 times higher the reaction rate than those of phenol with

formaldehyde.

Two steps are involved during formation of tannin-formaldehyde resins, methylolation and

condensation reaction. The first step involves methylolation, which is an electrophilic aromatic

substitution reaction. The second step is condensation reaction, where the methylol group of one molecule

reacts with a second tannin molecule, thus forming a methylene linkage. Due to high molecular weight

and complex structure of tannin, the process of cross linking is complicated, where resole resins produce

linear and regular crosslinking.

3.2 FTIR Spectroscopy

Figure 1 represents FTIR spectra of tannin (TA), tannin-formaldehyde resins (TFA), tannin-

aniline-formaldehyde resins (TAFA), tannin-formaldehyde resins adsorbed with arsenic (TFA-As), and

tannin-aniline-formaldehyde resins adsorbed with arsenic (TAFA-As). IR spectrum of tannin showed that

the absorption of broad band observed in the range of 3550 - 3100 cm-1

, is attributed to phenolic and

methylol –OH groups of tannin resins. The presence of absorption bands in the region of 1612 - 1447 cm-1

are characteristic of the elongation of aromatic -C=C- bonds. The absorption bands appeared in the range

of 1533 - 1447 cm-1

due to the deformation vibration of -C-C- bonds in the phenolic groups. The

absorption band observed at 1210 cm-1

is associated with the >C=O stretching of the benzene ring. The

presence of absorption bands in the region of 1085 - 1031 cm-1

is due to >C=O stretching and C-H

deformation.

IR spectrum of TFA resins showed small absorption bands observed in the range of 2800-2650

cm-1

are associated with methylene (-CH2-) bridges formed by reaction with formaldehyde [40-42]. It has

been demonstrated that large number of methylene ether bridges (-CH2-O-CH2-) occur, which rearrange

themselves to form methylene (-CH2-) bridges with release of formaldehyde [43]. The comparative study

of IR spectra of tannin and TFA resins shows that the peaks intensity of >C=O peak at 1117 cm-1

is

increased and also broadened. It indicates the formation of dimethylene ether (-CH2-O-CH2-) linkages by

the condensation of formaldehyde with tannin [44]. The intensity of absorption bands decreased in the

region of 827 - 760 cm-1

is due to the substitution at 6 and 8 positions of tannin. The presence of the band

in the region 1510- 1750 cm-1

which could be assigned to -C=C- stretching

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Figure 1. FTIR spectra of metal ion adsorbed on tannin resins. TA: Tannin; TFA: Tannin-

Formaldehyde resins; TFA-As: As(V) adsorbed on Tannin-Formaldehyde resins; TAFA: Tannin-

Aniline-Formaldehyde resins; TAFA-As: As(V) adsorbed on Tannin-Aniline-Formaldehyde resins.

observed. This could be due to the effect of metal ion chelating on the double bonds present in the

aromatic rings. Comparison of tannin, TFA, and TAFA with arsenic adsorbed spectra of TFA and TAFA

resins showed that the relative intensities of peaks at 1031-1713 cm-1

were changed because of metal-

tannate complex formation between arsenic species and phenolic groups of tannin [45].

3.3 Particle Size Distribution and SEM

The particle size distribution data of tannin-formaldehyde resins is presented in Table 3. It

illustrates that the particle size distribution is in the range of 0.8 - 1.71 μm. Representative particle size

distribution data was obtained by particle size analyzer (AccuSizer 780 Optical Particle Sizer, Santa

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Barbara, USA). Figure 2 depicts the SEM micrographs of the resin surface. Many small pores and

particles with diameter >3 μm diameter are observed on the surface of tannin resins.

Figure 2. SEM micrographs of tannin-aniline-formaldehyde (TAFA) resins.

3.4. Exchange Capacity of the Resins

The exchange capacity of tannin resins was determined by the proton exchange method [46].

Proton titration experiments were carried out to determine the number of valid sites for adsorption present

on adsorbent particles. The surface exchange capacity of tannin resins is reported in Table 3, which was

found to be in the range of 4.9 – 9.6 H+ mmol/g. The titration results showed that H

+ ion exchangeable

groups are largely –OH groups dominating the resin surface. The comparative study of TFA and TAFA

resins (Table 3) shows that TAFA resins have higher proton exchange values than TFA resins. It clearly

indicates that amino group of aniline with phenolic –OH groups of tannin improves proton exchange

capacity than that of phenolic –OH groups of tannin alone and it may be attributable to the synergistic

effect of additional amine functional group and phenolic –OH group of tannin within resins.

3.5 Adsorption Behavior of Metal Ion on Resins

The percentage adsorption of As(III) and As(V) at different pH is presented in Tables 4-7. In the

literature, different adsorption mechanism for the adsorption of metal ions on tannin based adsorbents has

been proposed. These include ion exchange, surface adsorption, chemisorptions, complexation and

adsorption-complexation [47-51]. It is commonly believed that ion exchange is mostly a pentavalent

mechanism. Metal ion forms bonding with –OH groups of tannin resins to release protons with their anion

sites to displace on existing metal.

Other studies have found evidence that tannin resins adsorb metal by complexation, surface

adsorption and chemisorptions [52] and concluded that the catechol or pyrogallol B ring offers special

opportunities for formation of metal complexes [45]. The results presented in Tables 5 and 6 show that the

binding of As(III) metal ions is almost 8-12 times higher than that of As(V). It is due to the proton

exchange capacity of As(III) which is higher than that of As(V). The results presented in Tables 6 and 7

show that TAFA resins adsorb higher amount of metal ions of As(III) than that of TFA resins.

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Table 4.As(V) binding on TAFA at different pH.

pH

TAFA

01

(mg/g)

TAFA

03

(mg/g)

TAFA

04

(mg/g)

TAFA

05

(mg/g)

TAFA

06

(mg/g)

TAFA

07

(mg/g)

TAFA

08

(mg/g)

TAFA

09

(mg/g)

TAFA

10

(mg/g)

2 24.1 20.0 17.5 24.6 24.5 23.1 22.3 23.6 24.9

4 23.7 23.7 24.9 24.8 25.5 22.5 24.1 24.9 24.8

8 24.7 24.6 23.9 24.8 25.1 23.0 23.9 24.7 24.7

9 23.9 24.5 20.8 23.7 25.3 203 24.1 23.7 24.6

10 23.7 24.4 21.7 24.2 24.7 24.5 24.7 19.5 24.2

Table 5. As(V) binding on TFA resins at different pH.

pH

TFA 01

(mg/g)

TFA 02

(mg/g)

TFA 03

(mg/g)

TFA 04

(mg/g)

TFA 05

(mg/g)

TFA 08

(mg/g)

TFA 09

(mg/g)

TFA 10

(mg/g)

2 21.6 15.2 24.7 24.1 24.4 24.6 25.1 25.1

4 22.3 21.6 23.9 24.4 24.6 23.7 24.8 24.5

8 22.2 14.9 24.1 24.7 24.6 24.5 24.8 24.9

9 22.7 13.0 23.4 24.0 24.0 23.5 24.2 24.0

10 20.6 5.0 23.2 23.8 24.0 24.7 20.5 24.2

Table 6. As(III) binding on TAFA resins at different pH.

pH

TAFA

01

(mg/g)

TAFA

03

(mg/g)

TAFA

04

(mg/g)

TAFA

05

(mg/g)

TAFA

06

(mg/g)

TAFA

07

(mg/g)

TAFA

08

(mg/g)

TAFA

09

(mg/g)

TAFA

10

(mg/g)

2 254.6 226.7 251.5 226.7 224.6 - 229.8 230.8 194.7

3 271.1 266.0 228.8 215.3 238.1 243.2 267.0 253.6 192.6

5 271.1 228.8 251.5 258.7 - 241.3 264.6 226.7 189.5

8 181.2 230.8 231.9 192.6 178.1 186.4 246.9 242.2 202.9

10 213.3 242.2 183.3 216.4 209.1 148.2 227.7 159.6 157.5

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Table 7: As(III) binding on TFA resins at different pH

pH

TFA 01

(mg/g)

TFA 02

(mg/g)

TFA 03

(mg/g)

TFA 04

(mg/g)

TFA 05

(mg/g)

TFA 08

(mg/g)

TFA 09

(mg/g)

TFA 10

(mg/g)

2 215.3 326.0 220.8 215.3 170.9 214.3 193.6 183.3

3 294.9 287.7 297.0 297.0 297.0 200.9 196.7 211.2

5 297.0 297.0 298.0 297.0 297.0 221.5 165.8 182.3

8 263.9 218.4 211.2 228.8 143.0 222.6 188.5 184.3

10 240.1 228.8 185.4 197.8 138.9 197.8 165.8 189.5

It is due to presence of aniline in TAFA resins, in which primary amino group of aniline increases

complex formation due to lone pair of electrons present on nitrogen. It is also due to the synergistic effect

of additional amine group present in aniline and phenolic –OH groups within tannin resins. Adsorption of

As(III) binding on TAFA and TFA resins shows that adsorption increases with increase in formaldehyde

concentration (Tables 6 and 7). To make a tannin resin insoluble in both acidic and basic media, tannin is

crosslinked with formaldehyde so as to more and more tannin molecule attached together. Due to

crosslinking of tannin with formaldehyde more surface hydroxyl groups are available for metal chelation.

In order to explain the adsorption mechanism, FTIR spectroscopy (Fig. 1) and Energy-dispersive

X-ray spectroscopy (EDX) (Fig. 3) studies were carried out. IR spectrum of TFA-As adsorbed resins

revealed that the absorption of methylol groups (-C-O) in the region of 1170 - 1050 cm-1

decreased in

intensity and broadened the peak than those of tannin and TFA resins spectra. EDX spectrum also

confirmed arsenic adsorption on tannin resins.

Figure 3. EDX spectrum of metal adsorbed on tannin (TAFA) resins.

3.6 Effect of pH

The effect of pH on adsorption of As(III) and As(V) were studied in the range of 2-10. The

percentage adsorption of As(III) and As(V) on tannin resins at different pH is presented in Tables 4-7. It

shows that the adsorption of arsenic (III) or (V) is independent on pH. This is attributable to the arsenic

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solutions formed in the aqueous which are subsequently adsorbed on the resins surface.

At lower pH, the functional groups of tannin resins are protonated to a higher extent and such a

condition is not favorable for As(III) removal resulting almost no change in the extend of adsorption

within the pH range 2-5 [53]. Arsenic present in natural waters in two common forms: arsenite (AsO33-

)

and arsenate (AsO43-

) are referred to arsenic (III) and arsenic (V) respectively. Smedley and Kinniburgh

[1] demonstrated that arsenate is the dominant form of arsenic in oxidized environment, whereas arsenite

dominates in reduced system. Pentavalent arsenates exists in different species namely H3AsO4, H2AsO4-,

HAsO3-2

, AsO4-3

in the pH range of <2, 3-6, 8-10, and >12 respectively. Trivalent arsenites include

As(OH)3, As(OH)4-, AsO2OH

-2, and AsO3

-3. As(V) species predominates and stable in oxygen rich aerobic

environments while trivalent arsenates predominates in moderately reducing anaerobic environments such

as ground water [54]. It is thus considered that the metal anions are adsorbed by releasing protons from

phenolic –OH groups of tannin in the pH range of 3-5 according to anion exchange mechanism.

3.7 Effect of Contact Time

Figures 4 and 5 show the effect of contact time for the adsorption of As(V) and As(III) on tannin-

formaldehyde resins respectively. It is evident that time has a significant influence on the adsorption of

As(V). It was observed that the percentage removal of As(V) from aqueous solution increases rapidly and

reaches up to 83% within 10 min. After that, the percentage removal of As(V) increases slowly and

reaches up to 86% in a total period of 30 min. A further increase in contact time has negligible effect on

the percentage removal of As(V). In the case of percent removal of As(III) the rate of adsorption is lower,

and it takes almost 30 min. to reach 90% adsorption. Later on adsorption becomes almost steady. It may

be due to the saturation of adsorption capacity of tannin resins.

The mechanism of solute transfer to the solid includes diffusion through the film around the

adsorbent particle and diffusion through the pores to the internal adsorption sites. Initially the

concentration gradient between the film and the solid surface is large, and hence the transfer of solute

onto the solid surface is faster. Thus, it takes lesser time to attain 83 percentage removal of As(V). As the

time increases, intraparticle diffusion becomes predominant. Thereby, solute takes more time to transfer

from solid surface to internal adsorption sites through the pores [55].

Figure 4: Effect of contact time for the adsorption of As(V) on

tannin-formaldehyde resins

0

20

40

60

80

100

0 5 10 15 20 25 30 35

Time (Min)

% A

dso

rpti

on

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Figure 5. Effect of contact time for the adsorption of As(III) on

tannin-formaldehyde resins

3.8 Adsorption Kinetics

To evaluate sorption dynamics, it requires consideration of two important physico-chemical

parameters such as kinetics and equilibrium of adsorption. Kinetics describing the solute adsorption rate

which governs the contact time. The study of equilibrium is the determining distribution of the solute

between solid-liquid phases and determining feasibility and capacity of the sorbent for adsorption. Several

kinetic models currently in use to explain the mechanism of adsorption progress are of the most simple

and widely used is pseudo-first order equation of Lagergren [56].

The qe is the mass of metal adsorbed at equilibrium (mg/g), qt is mass of metal adsorbed at time t

(mg/g) and Kf is the first order reaction constant (L/min). The pseudo-first order kinetics considers the

rate of occupation of adsorption sites is proportional to the number of unoccupied sites. A graph of log

(qe-qt) verses t indicates the application of the first order kinetic model (Figures 6 and 7).

Figure 6. Pseudo-first order reaction for As(V).

0

20

40

60

80

100

0 25 50 75 100 125 150 175 200

Time (Min)

% A

ds

orp

tio

n

Ca




Figure 7. Pseudo-first order reaction for As(III).

On the other hand, equilibrium capacity may be expressed by pseudo-second order equation as

follows:

Where, K2,ad is the second order reaction rate equilibrium constant (g/mg.min). A plot of t/qt versus t give

a linear relationship for the applicability of the second order kinetic (Figures 8-9).

Figure 8. Pseudo-second order reaction for As(V).

Figure 9. Pseudo-Second order reaction for As(III)

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3.9 Desorption Study

Reuse of tannin-formaldehyde resins is an important step in order to make the adsorption process

more economical. The desorption of arsenic from the tannin resins may be the reverse process of

adsorption, in which the arsenic adsorbed onto the solid surface of tannin resins released to the aqueous

phase. For desorption study, various eluents such as HCl, HNO3, NaCl and NaNO3 are used. The

regeneration efficiency of tannin resins at 1M HNO3 solution was studied. In order to study the reusability

of tannin resins, the adsorption/desorption cycles of As(III) and As(V) were repeated three times by using

same adsorbent. When HNO3 was used as a desorbing agent, the tannin resins surface was completely

covered by H+ ions. The coordination spheres of chelated As(III) and As(V) ions could not compete with

H+ ions for desorption sites and subsequently heavy metal ions were released from solid surface into the

solution. Nearly 95% of the adsorbed As(III) and As(V) ions were desorbed from tannin resins by using

1M HNO3 solution in the first step, which then decreased to 85% at the third step.

4. CONCLUSION

The present study demonstrated the use of tannin-formaldehyde and tannin-aniline-formaldehyde

based resins as adsorbents for As(III) and As(V) metal ions. Tannin-formaldehyde resins were prepared by

condensation reaction between tannin and formaldehyde, whereas tannin-aniline-formaldehyde resins

were prepared by condensation of tannin and aniline with formaldehyde. Synthesized tannin-

formaldehyde resins are insoluble in water, whereas the natural tannin is soluble in water. Tannin-

formaldehyde resins were characterized by FTIR and EDX before and after adsorption of the metal ions.

The batch experiments were studied as a function of particle size, initial pH, proton exchange capacity

and initial concentration of As(III) and As(V). Equilibrium and kinetic studies were conducted for the

adsorption of As(III) and As(V) from aqueous solution on to tannin resins with the concentration of 25

mg/g for As(V) and 200 mg/g for As(III) at pH 5. The adsorption mechanism may be partly results of the

ion exchange or complexation between arsenic ion and phenolic –OH groups on tannin resin surfaces.

Chemically modified tannin resins have been proposed to be an efficient and economical alternative

sorbent in arsenic ion removal from water.

ACKNOWLEDGMENT

The authors would like thanks to the Department of Science and Technology, New Delhi, India

for the generous support of this research under the Program No. SR/S3/CE/0049/2010. The authors also

would like to thanks to Dr. S. Ponrathnam for constant encouragement.

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The authors declare no conflict of interest

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