Adsorption of Abietic Acid from Colloidal Suspension by Calcined Mg

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Adsorption of Abietic Acid from Colloidal Suspension by Calcined Mg/Al Hydrotalcite Compounds Dongmei Yang, Zhanqian Song, and Xueren Qian Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie303513n • Publication Date (Web): 06 May 2013 Downloaded from http://pubs.acs.org on May 6, 2013

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Industrial & Engineering Chemistry Research

1

Adsorption

of

2

Suspension

by

3

Compounds

Abietic

Acid

Calcined

from

Mg/Al

Colloidal

Hydrotalcite

Dongmei Yang†,‡, Zhanqian Song §, and Xueren Qian*,†

4 5

†

6

Northeast Forestry University, Harbin, Heilongjiang Province, 150040, P. R. China.

7

‡

8

Guangzhou, Guangdong Province, 510641, P. R. China.

9

§

Key Laboratory of Bio-based Material Science and Technology of Ministry of Education,

State Key Lab of Pulp and Paper Engineering, South China University of Technology,

Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, Nanjing,

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Jiangsu Province, 210042, P. R. China.

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* Corresponding author. Tel.: +86 451 82192081; fax: +86 451 82192081. E-mail address:

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[email protected] (Xueren Qian).

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ABSTRACT: Resin acids are major constituents of wood extractives and are known to cause a

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range of paper manufacturing problems including pitch deposition, poor paper machine

15

runnability, and low paper quality. Abietic acid (AA) is one of major constituents of resin acids.

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Removing AA from colloidal suspension using calcined Mg/Al hydrotalcites (CHTs) was

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investigated in this study. The removal percentage of AA at the pH value of 6.8 was larger than

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those at the pH values of 7.8-9.8 and the adsorption capacity was 434.78 mg/g at 323 K. Kinetic

2

data were followed pseudo-second-order kinetic model and adsorption followed the Langmuir

3

isotherms. Adsorption process proceeded by physical adsorption. About 80% of the equilibrium

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adsorption capacity of the original CHT was achieved after 5 use cycles. The calcined Mg/Al

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hydrotalcites exhibited high adsorption capacity for AA, and were efficient adsorbents for

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dissolved and colloidal substances (DCS) in papermaking process.

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1. INTRODUCTION

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In modern papermaking, the process water is re-circulated extensively within internal loops to

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use less of fresh water and to make the papermaking process more environmentally friendly.

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This will inevitably lead to an enrichment of any substances released from the pulp into the

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water.1 These substances released during the pulping and bleaching operations are commonly

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divided into dissolved and colloidal substances (DCS), which have an anionic character and are

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often referred to as “anionic trash”. The DCS increased cause a range of detrimental problems in

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paper products and paper machine operations, including “pitch” deposition, decreased paper

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machine runnability and impairment of paper strengths and optical properties.2,3 Productivity

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losses can result in economic losses as high as 10–20%.4 The DCS are mainly made up of

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hemicelluloses, pectin, lignin and lignin-like oligomers, wood resin such as resin acids in

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suspension of unbleached spruce TMP.5 Resin acids are a class of lipophilic wood extractives,

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and are the most important substances causing pitch problems and affecting paper properties.

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Therefore, it is particularly important to reduce the content of resin acids in the white water.

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The most common way of handling pitch is to retain the pitch in the paper sheet by

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papermaking fillers such as calcium carbonate,6 talc,7,8 Kaolin,8,9 bentonite,10 and remove it from

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the white water system. The second approach is to use anionic dispersants to keep the pitch


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molecules from deposition on the machine.11 The disadvantages of the above two methods are to

2

decline quality of paper products and also increase problems when paper is recycled. The

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dissolved air flotation (DAF) system was used to overcome pitch disturbances and improve pulp

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quality.12 In addition, handling pitch by various enzymes13–16 was frequently reported. However,

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the above listed methods of pitch control were all disposable because the materials used cannot

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be recycled.

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Hydrotalcites (HTs), also known as layered double hydroxides (LDHs), or anionic clays,

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consist of brucite-like hydroxide sheets, where partial substitution of trivalent for divalent

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cations results in a positive sheet charge compensated by anions within interlayer galleries.17

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Hydrotalcites calcined at 500–800 oC (CHTs) are transformed to the Mg–Al mixed oxides those

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are capable of adsorbing anions from aqueous solutions through the reconstruction of their

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original layered structure (called “memory effect”).18 The anion contaminants adsorbed from

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water solution or suspension by CHTs have been reported and showed high adsorption capacity.

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The adsorbed anion materials mainly included acids,19,20 heavy metal ions,21,22 pesticides,23–27

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dyes,28–30 halogens,31,32 phosphates,33–35 surfactants,36 etc.

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Abietic acid (AA) is a compound of resin acids in lipophilic wood extractives, its content was

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shared one-third of DCS from thermo-mechanical pulp.37 AA was showed to be the best marker

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for total resin acid content of in-mill process water samples of a TMP/CTMP pulp mill,38 and its

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content in water is easy to be determined such as by UV-Vis analysis.8,39 The presence form of

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AA is anion or negatively charged colloid in the white water.40 Whatever form of existence, it

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can be adsorbed by memory effect or electrostatic attraction using the CHT adsorbent. Hence,

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AA was selected as a model compound of resin acids, and the CHTs were used as the adsorbent

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in this study. The effects of pH, initial AA concentration and contact time on adsorption behavior


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were examined, and the adsorption kinetics and thermodynamics of removal of AA by CHTs

2

were studied.

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2. EXPERIMENTAL SECTION

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2.1. Materials. Mg(NO3)2·6H2O, Al(NO3)3·9H2O, NaOH and Na2CO3 were of AR grade and

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used as received without purification. Gum rosin was obtained from Shanghai Sanlian Industrial

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Co., Ltd, China. All the water used was deionized. Hydrotalcites with Mg/Al molar ratios of 2:1,

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3:1 and 4:1 were prepared by coprecipitation at a constant pH of 10. The samples were heated at

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500 oC for 5 h in a muffle furnace to prepare the calcined hydrotalcites (CHTs). The zeta

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potential of the CHT particle with Mg/Al molar ratio of 2:1 determined by a zeta potential

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analyzer (Zeta PALS, Brookhaven Instruments Corp., NY) was 14.5 mV in water at pH 6.8, and

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decreased with increasing pH (the value was close to zero at pH 9.8). The preparation and

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characterization details of the sorbent materials can be found in reference.19

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2.2. Purification of AA. 100 g of gum rosin was dissolved in 300 mL of 80% ethanol solution

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by heating at 80 oC. The solution was cooled in room temperature after the gum rosin was

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completely dissolved. The white suspension appeared while deionized water was added into the

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solution. The resulted suspension was allowed to separate into layers, and the supernatant was

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discarded. The precipitate was dissolved into the ethanol solution again. This process was

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repeated until the supernatant was clarified. A large amount of precipitate was obtained after a

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certain amount of 0.5 mol/L hydrochloric acid solution was added into the suspension. The white

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precipitate was dried at 65 oC for 24 h. Part of the sample was detected by GC/MS,41 and the

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results showed the abietic acid content is 73.37%.

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2.3. Preparation of AA colloidal suspensions. AA colloidal suspension was prepared by first

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dissolving AA sample in 20 mL ethanol, and then dispersing this solution in 1 L deionized water.


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The resulted colloidal particles had a diameter of 198 nm and a zeta potential of -15.3 mV at pH

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6.8, respectively, as determined by a zeta potential analyzer (Zeta PALS, Brookhaven

3

Instruments Corp., NY).

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2.4. Adsorption experiments. The adsorption experiments were carried out in 50 mL sealed

5

centrifuge tube by mixing a 30 mL AA suspension of appropriate concentration and 0.02 g

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adsorbent and shaking in the constant temperature water bath. Suspension system was vibrated

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for 24 h unless otherwise stated. The initial pH value of AA suspension was adjusted by addition

8

of 0.1 mol/L HNO3 or KOH solution. After adsorption, the mixture was centrifuged for 40 min at

9

5000 rpm, and the supernatant was analyzed for the presence of AA by UV–Vis

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spectrophotometry.

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The concentration of AA was determined in UV–Vis spectrometer (model TU-1901, Beijing

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Purkinje General Instrument Co., Ltd., China) at 242 nm.6 The calibration graph of absorbance

13

versus concentration obeyed a linear Beer–Lambert relationship. The effect of pH of the AA

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solution/suspension over the calibration curve was studied, and the standard curve equation

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(absorbance (A) versus concentration (mg/L)) was obtained as follows: y=0.0528x-0.0336,

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R2=0.9996 (pH 6.8); y=0.0561x-0.0238, R2=0.996 (pH 7.8); y=0.047x+0.0504, R2=0.993 (pH

17

8.8); y=0.0512x+0.017, R2=0.995 (pH 9.8).

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2.5．． Sorbent recycling. The possibility of recycling the CHT was evaluated by repeating

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adsorption/calcination experiments. The method was followed as suggested by Crepaldi et al.42

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The adsorption conditions were as follows: AA concentration 260 mg/L, pH 6.8, adsorbent

21

concentration 0.67g/L, and contact time 24 h.

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2.6. Calculations. The amount of AA adsorbed by the CHT at a specific time t was defined as follows (Eq (1)):


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1

（c - ct）V qt = 0 m

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(1)

2 3 4

The removal percentage of AA was calculated using the following equation (Eq (2)):

c −c Removal (%) = 0 t × 100 c0

(2)

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where qt is the amount of adsorbed AA at time t (mg/g); V is the suspension volume (mL); c0 and

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ct are the initial and remaining AA concentrations in suspension before adsorption and at time t

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(mg/L), respectively; and m is the mass of the adsorbent (g).

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In all cases, the adsorbents can reach equilibrium adsorption capacities within 24 h. Hence, the

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amount of AA adsorbed by the CHT at time 24 h was defined as the equilibrium adsorption

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capacity (qe, mg/g) in this study.

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3. RESULTS AND DISCUSSION

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3.1. Effect of initial pH on adsorption. Generally, the pH is an important variable, which

13

controls the adsorption at water-adsorbent interfaces. Resin acids are hydrophobic carboxylic

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organic acids that change in solubility with dissociation as a function of pH. Increasing solubility

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of resin acids with pH corresponds also to a decrease in hydrophobicity.43 The extent of

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dissociation of weak acids with pH is predicted by their pKa values. Due to the facts that the pKa

17

value of AA is 6.444 or 7.245 and the pH of papermaking tends to neutral or slightly alkaline,46 the

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investigated AA suspension initial pH range was determined from 6.8 to 9.8. The adsorptions of

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AA on the CHTs at different initial pH values are depicted in Figure 1. From the figure it was

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observed that the removal percentage of AA by the CHTs reached maximum at pH 6.8, and

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decreased with the increase of initial pH. This can be attributed to the increasing competition

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between OH− groups and AA species for binding to the adsorption sites, resulting in decreasing

23

sorption capacity and removal of the AA. The zeta potential of the abietic acid droplets increased


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from -15.3 to -1.7 mV with increasing pH from 6.8 to 9.8. Meanwhile, the zeta potential of the

2

CHT deceased from 14.5 mV to zero. Therefore, the electrostatic attraction of CHT on the

3

abietic acid decreased with increasing pH, resulting in the decrease of removal percentage.

4

In addition, it was observed from Figure 1 that the removal percentage of AA decreased with

5

increasing Mg/Al molar ratio. This could be due to the fact that CHT with Mg/Al molar ratio of

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2:1 contains a higher amount of Al3+ and therefore the net positive charge on the hydroxide layer

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is higher compared to the other samples. Similar observations have been made for selenium47

8

and phosphate33 adsorption on Mg/Al HT. On the other hand, the surface areas of the CHTs

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given in our previous work were 127.67 (2:1), 150.38 (3:1), and 157.91 m2/g (4:1).19 The

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removal percentage of AA decreased with increasing surface area of the CHTs, which might be

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implied that the structure reconstruction of the CHT played a major role in the adsorption.

12 13

The CHT with Mg/Al molar ratio of 2:1 and initial pH of 6.8 were chosen for subsequent study.

100 90 Removal(%)

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80 2:1 3:1 4:1

70 60 50 6

14

7

8

9

10

11

pH


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Figure 1. Effect of the initial pH on AA uptake by CHTs with different Mg/Al molar ratios.

2

(c0=260 mg/L, CHT concentration 0.67 g/L).

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3.2. Effect of contact time on adsorption. The effect of contact time on the amount of AA

4

adsorbed onto adsorbent materials was also investigated at 298 K and the result is shown in

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Figure 2. The curves showed that the adsorption quantity on the CHT increased rapidly in the

6

first 1 hour, almost 95 percent removal of the AA by the adsorbent occurred in this period. There

7

was an increase in the concentration of AA adsorbed, but at slower rate, until 1.5 hours, and no

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significant change in the adsorption amount with further increase in contact time.

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Kinetic modeling not only allows estimation of sorption rates but also leads to suitable rate

10

expressions characteristic of possible reaction mechanisms. In this respect, several kinetic

11

models including the pseudo-first-order equation (Eq. (3)),48 pseudo-second-order equation (Eq.

12

(4)) 48 and intraparticle diffusion model (Eq. (5)) 49 were analysed.

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1 k1 1 1 = × + qt q e t q e

(3)

t 1 1 = t+ qt q e k 2 qe 2

(4)

qt = k p t 1 2 + C

(5)

17 18

Where, qe and qt (mg/g) are the adsorption quantities on the adsorbent at equilibrium and at

19

time t (min), respectively; k1 (min−1) and k2 (g/(mg·min)) are the rate constants of pseudo-first-

20

order and pseudo-second-order kinetic models, respectively; C is the intercept (mg/g) and kp is

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the intraparticle diffusion rate constant (mg/(g·min1/2)).

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Both parameters and the correlation coefficients are given in Table 1. From the table, it could

23

be seen the correlation coefficient R2 of pseudo-second-order expression was much higher than


8

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those of pseudo-first-order expression and intraparticle diffusion model. Therefore, the

2

adsorption of AA by CHT conforms to the pseudo-second-order kinetic model.

180

q t (mg/g)

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160

140

120 0

2

4

6

Time (h) 3 4

Figure 2. Effect of contact time on the uptake of AA by CHT. (pH=6.8, c0=150 mg/L, sorbent

5

concentration 0.67 g/L, adsorption temperature 298 K).

6 7

3.3. Effect of initial AA concentration on adsorption. The equilibrium adsorption quantities

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of AA on the CHT as a function of equilibrium concentrations of AA are illustrated in Figure 3.


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The equilibrium adsorption quantities increased considerably with an increase in the lower initial

2

concentrations. The equilibrium sorption experimental data obtained in this study were analyzed

3

using the commonly used Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm

4

models.

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The Langmuir isotherm model 50 is described by the following equation (Eq. (6)):

6

qe =

Q max bc e 1 + bc e

(6)

7

where qe is the adsorption quantity on the adsorbent at equilibrium (mg/g), Qmax is the maximum

8

sorption quantity (mg/g), ce is the equilibrium concentration of AA in the suspension (mg/L), and

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b (L/mg) is the Langmuir constant related to the adsorption energy. For convenience of plotting

10

and determining the Langmuir constants, the Langmuir equation can be rearranged to linear form

11

as below (Eq. (7)):

12

ce ce 1 = + q e Q max Q max b

(7)

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The essential characteristics of the Langmuir isotherm can be expressed by a dimensionless constant called equilibrium parameter, r defined by (Eq. (8)): 51

r=

1 1 + b × c0

(8)

17

where b is the Langmuir constant and c0 is the initial AA concentration (mg/L), r values indicate

18

the type of isotherm. An r value between 0 and 1 indicates favorable adsorption.52 The r values

19

at 298 K, 318 K and 323 K were found to be 0.068, 0.072 and 0.069, respectively. These values

20

indicated that the adsorption is favorable.

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The Freundlich isotherm model 53 is described by the following equation (Eq. (9)):

22

qe=Kce1/n


(9)

10

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1

where qe is the adsorption quantity on the adsorbent at equilibrium (mg/g), ce is the equilibrium

2

concentration of AA in the suspension (mg/L), K ((mg/g)/( mg/L)1/n) and n are the Freundlich

3

temperature-dependent constants.

4

Note that Eq. (9) is often used in the linearized form (Eq. (10)): 1 n

5

lnqe= lnce+lnK

(10)

6

The apparent equilibrium constants of Freundlich isotherm equations and Langmuir isotherm

7

equations were evaluated at different temperatures and are given in Table 2. Based on the

8

correlation coefficients (R2), the adsorption data were much better fitted with the Langmuir

9

equation than the Freundlich equation. Therefore, the adsorption process was considered to obey

10

the Langmuir isotherm.

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It is known that the Langmuir and Freundlich isotherm constants do not suggest anything

12

about the adsorption mechanism. In order to understand the adsorption type and the mechanism,

13

the equilibrium data were applied to Dubinin–Radushkevich (D–R) isotherm model 54 (Eq. (11)):

14

ln qe ＝ ln Qmax－β× εe2

(11)

15

where β is a constant related to the mean free energy of adsorption per mole of the adsorbent

16

(mol2 /kJ2), Qmax is the theoretical saturation capacity (mg/g), and εe is the Polanyi potential,

17

which is equal to RTln(1+(1/ce)), where R (J/(mol·K) is the gas constant, and T (K) is the

18

absolute temperature, ce is the equilibrium concentration of AA in the suspension (mg/L).

19

The plot of lnqe vs. εe 2 results a straight line as shown in Figure 4. The numerical values of D-

20

R constants β and Qmax were evaluated from the intercept and the slope of the graph and are

21

given in Table 2. The mean free energy of adsorption (E), defined as the free energy change

22

when one mole of AA is transferred to the surface of the solid, from infinity in suspension, was

23

calculated from the β-value using Eq. (12).


11


E= (2β) −0.5

1

(12)

360 320 q e (mg/g)

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280 293K 308K

240

323K

200 20

40

60

80

100

120

c e (mg/L) 2 3

Figure 3. Effects of initial AA concentration and temperature on the adsorption capacity of CHT.

4

(pH = 6.8, adsorption time = 24 h).

5

If the magnitude of E is between 8 and 16 kJ/mol the adsorption process proceeds by ion

6

exchange, while for values of E

Adsorption of Abietic Acid from Colloidal Suspension by Calcined Mg

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