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Starch/Stärke 58 (2006) 391–400

DOI 10.1002/star.200600487

Amal A. Aly

Preparation, Characterization and Evaluation of Anionic Starch Derivatives as Flocculants and for Metal Removal

National Research Center; Textile Division, Dokki Cairo, Egypt

391

Anionic starch derivatives bearing carboxyl groups were prepared by reacting maize starch and hydrolyzed maize starch with a previously synthesized citric acid adduct (2-hydroxy-3-chloropropylcitric acid) using the dry process. Different factors affecting the preparation, i.e. catalyst concentration, citric acid adduct concentration, duration and temperature have been studied. The modified starch was characterized by IR spectroscopy, X-ray diffraction analysis and scanning electronic microscopy. The anionic starch derivatives were evaluated as flocculants. Different factors affecting the flocculation efficiency of the prepared flocculants were studied. These factors include effect of dose, acid content, pH of the flocculation medium and molar mass. The anionic starch samples were chelated with different metal cations such as Cu21, Co21, Mg21, and Hg21, in the form of acetates, chlorides and sulfates.

1 Introduction Anionic flocculants are very useful for sedimentation of copper flotation tailings [1], fine clay particles [2], wastewaters of porcelain manufacture [3], phosphatic clay waste [4], and for selective flocculation in the dolomiteapatite system [5]. Anionic polyelectrolyte flocculants have been prepared by polymerization of acrylic acid [6] or copolymerization of acrylamide with comonomers bearing acidic groups [7]. Environmental pollution caused by toxic heavy metals in industrial effluents is one of the most severe problems in many densely populated cities worldwide. Under the current regulations in most countries, industries are obligated to treat wastewater and reduce toxic metal concentration below certain limit values. The use of synthetic resins for chelating toxic metal ions in wastewater is a possible approach for preventing environmental pollution and recycling metals. In general, after the adsorption of metal ions, the chelating resins are discarded in landfills or treated by incineration. However, these processes often result in secondary environmental pollution by contaminating the soil or air. In addition, these synthetic polymers are usually non-renewable and non-biodegradable.

Correspondence: Amal A. Aly, National Research Center; Textile Division, Dokki, Cairo, Egypt. E-mail: [email protected].

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Starch, a biopolymer, is more attractive for industrial use because of its renewability, biodegradability, and low cost. However, starch by itself has inherently no flocculating or chelating or metal-interaction capacity. Hence, several approaches have been made to utilize starch as a flocculant and metal scavenger by introducing substituents with flocculating and chelating activity, such as carboxylate. The carboxylation has been achieved by reaction of starch with monochloroacetic acid [8, 9], saponification of poly(acrylonitrile)-starch graft copolymer, poly(acrylamide)-starch graft copolymer, or poly(methylacrylate)-starch graft copolymer; by graft polymerization of starch with acrylic or methacrylic acid [10–14], or application of poly(methylacrylate)-pregelled starch graft copolymer [15]. Polymeric flocculants based on biopolymers such as guar gum, alginate, starch [16] and dextrins [17] have been prepared. Some anionic starch derivatives were synthesized and tested as flocculants [18]. The aim of this work was to prepare and characterize anionic starch derivatives bearing carboxyl groups and evaluate their potential as flocculant and metal removal. This is to clarify the different factors affecting the preparation, these factors include the catalyst concentration and the citric acid adduct concentration, duration and temperature. Also different factors affecting the flocculation efficiency of the prepared flocculants were studied, encompassing effect of dose, acid content, pH of the flocculation medium and molar mass. The anionic starch samples were chelated with www.starch-journal.com

Research Paper

Keywords: Flocculation; Anionic starch; Hydrolyzed starch; Starch ethers; Metal removal

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different metal cations such as Cu21, Co21, Mg21, and Hg21, supplied as acetate, chloride and sulfate. Further, the modified starches were characterized.

2 Materials and Methods

Starch/Stärke 58 (2006) 391–400

2.3 Preparation of flocculant solution The flocculant solution was prepared by boiling the anionic starch derivative (1 g) in 90 mL distilled water at 1007C until complete solution and clarity was obtained. Then the total volume of the solution was adjusted to 100 mL with distilled water.

2.1 Materials Lauric acid, sodium hydroxide, ferric chloride, epichlorohydrin, citric acid, magnesium acetate, magnesium chloride, magnesium sulfate, copper acetate, copper chloride, copper sulfate, cobalt acetate, cobalt chloride, cobalt sulfate, mercuric acetate, mercuric chloride and mercuric sulfate were of analytical grade (Merck, Poole, UK). Maize starch was supplied by Cairo Company for Starch and Glucose. Three types of hydrolyzed starches, namely H1-, H2- and H3-starch were prepared by HCl treatment of starch at 507C for 2 h using a liquor ratio of 5. The acid concentrations were 0.5, 0.75 and 1 M for preparation of H1-, H2- and H3-starch, respectively. H1-, H2-and H3starch have intrinsic viscosity [Z] values of 1.6, 0.8 and 0.6, respectively. Starch and hydrolyzed starches have been used in further chemical modification to produce different anionic starches. 2-Hydroxy-3-chloropropylcitric acid (citric acid adduct) was prepared by reaction of 1 mol epichlorohydrin with 0.9 mol of monosodium citrate under reflux until one phase was formed. After partial cooling to 60–707C, the reaction product was poured into a large excess of 96% ethanol in a vessel to remove unreacted epichlorohydrin and formed glycols. The prepared compound was filtered and dried.

2.2 Preparation of starch-2-hydroxypropylcitric acid The anionic starches were prepared by reacting starch with 2-hydroxy-3-chloropropylcitric acid (citric acid adduct) in presence of sodium hydroxide using the dry technique according to a reported method [19]. Native or hydrolyzed starches was thoroughly mixed with powdered sodium hydroxide (using the NaOH: citric acid adduct molar ratio) for 5 min using mechanical stirring. The citric acid adduct (X mol: anhydroglucose unit) was added to the previous mixture at room temperature and thoroughly mixed for 5 min then transferred to a stoppered bottle and kept in a thermostated water bath at different temperatures for different periods of time. At the end of the reaction time the product was poured onto 200 mL ethanol and the pH was adjusted to 8, filtered, Soxhlet extracted for 12 h using an ethanol: water mixture (80:20, v/v) and finally dried.


2.4 Preparation of ferric laurate Lauric acid (5 g) was suspended in 500 mL water in a 1-L beaker, stirred at 60–707C and an aqueous solution of sodium hydroxide (10%, w/v) was added dropwise to the acid up to pH 10.5. Stirring was continued till the fatty acid salt dissolved completely. The ferric laurate was prepared by adding aqueous ferric chloride (10%) dropwise to the above solution. The pH was adjusted to 6 and 8 by controlling the addition of the ferric chloride solution. The suspension was then transferred to a 1-L measuring flask and 30 mL of ferric laurate solution were diluted to 1 L.

2.5 Flocculation process The flocculant solution was added in different doses to a series of 250-mL beakers containing 100 mL of ferric laurate solution and the mixtures were mechanically stirred at 200 rpm for 30 s. After 10 min the solution was filtered. The transmission% of the filtrate was measured using the spectronic 20D1 apparatus (Spectronic Instruments, Rochester, NY, USA) and the dry weight of the precipitate was determined. Weight removal % ¼ Weight of precipitate 100 Solid content of 100 mL ferric laurate solution

2.6 Removal of different metal cations The anionic starch samples were chelated with different metal cations (Cu2, Co21, Mg21, and Hg21). A solution (50 mL) of a metal ion (0.01 M) was treated with the prepared starch sample (0.25 g) added as solid. After 24 h at room temperature the solutions were filtered and the residual metal concentrations in the filtrate were measured by atomic absorption spectroscopy (Spectrometer 175, Instrumentation Lab., aa/ae, Lexington, MA, USA). Removal [%] =

Decrease in cation concentration ½meq 100 Cation exchanger capacity ½meq=100g

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Starch/Stärke 58 (2006) 391–400

Anionic Starch Derivatives as Flocculants and for Metal Removal

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2.7 Testing and analysis – The carboxyl content and chlorine content were determined according to reported methods [20] and [21], respectively. – Infrared (IR) spectra were recorded in a Bruker IR Spectrometer (Bruker, Rheinfelden, Germany). – X-ray analysis was carried out using a Diano Corporation X-ray diffractometer (Medford, MA, USA) equipped with cobalt Ka radiation using iron filter. The diffraction angle range was 5–807 with a step size 0.17 2U and scanning time per step 1 s). – Scanning electron micrographs were obtained in a scanning electron probe microanalyzer (JXA-840A, JEoL, Tokyo, Japan). The specimens in the form of films were mounted on the specimen stubs and coated with a thin gold film by sputtering. The micrographs were taken at magnification of 1000 using 10 kV accelerating voltage.

3 Results and Discussion Cl – CH2 – CH – CH2 – R + NaOH

CH2 – CH – CH2 – R + NaCl

OH

O

(Citric acid adduct) St – OH + CH2 – CH – CH2 – R

NaOH

St – O – CH2 – CH – CH2 – R

O

CH2 – CH – CH2 – R + H2O

(1)

OH

CH2 – CH – CH2 – R (2)

O

OH

St – O – CH2 – CH – CH2 – R

OH

St – OH + CH2 – CH – CH2 – R (3)

OH

OH

OH

CH2 – COOH HO – C – COOH = R CH2 – COOH

(Citric acid)

Fig. 1. Effect of sodium hydroxide concentration on the reaction of starch with citric acid adduct Starch, 11 mmol; citric acid adduct, 8.25 mmol; at 707C for 60 min.

ried out at 707C for 60 min, using 11 mmol starch, a citric acid adduct : starch molar ratio 1:1, and different sodium hydroxide: citric acid adduct molar ratio (0.0625– 0.75). As can be seen an increasing NaOH concentration up to a molar ratio of 0 125 causes significant enhancement in the extent of the reaction. Thereafter, further increase in the alkali concentration decreases the yield. That is, a maximum yield is obtained at a NaOH molar ratio of 0.125. It is logical that lower NaOH concentrations are not sufficient to drive the reaction to its maximum, however, higher NaOH concentration favor alkaline hydrolysis of the functional group of the citric adduct and/or splitting off of the citric adduct moieties from the starch by alkaline hydrolysis. Thus, it can be concluded that NaOH concentrations determine the extent of the desirable reaction (reaction 1) and the undesirable side reactions (reactions 2 and 3) as well.

3.1 Preparation of anionic starch 3.1.1 Sodium hydroxide concentration

3.1.2 Citric acid adduct concentration

Fig. 1 shows the dependence of the extent of the reaction of starch with the citric acid adduct (expressed as carboxyl content) on the NaOH concentration. The reaction was car-

Fig. 2 shows the carboxyl content, an indication of the extent of the reaction, as a function of citric acid adduct: starch molar ratio (0.25– 1.5).



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Fig. 2. Effect of citric acid adduct concentration on the reaction of starch with citric acid adduct. Starch, 11 mmol; molar ratio sodium hydroxide: citric acid adduct concentration, 1:8; at 707C for 120 min.

The extent of the reaction increases sharply as the citric acid adduct: starch molar ratio increases up to 0.75, at higher ratios the carboxyl content decreased. This could be interpreted in terms of shortage of accessible starch hydroxyl groups which are available for reaction with the citric acid adduct. It is also possible that the starch molecules undergo changes that diminish their susceptibility towards further reactions and that the citric acid adducts at higher concentrations are more susceptible to alkaline hydrolysis. All these reactions would balance the sharp increase observed upon using citric acid adduct at concentrations up to 0.75 molar ratio.

starch (11 mmol) was reacted with 8.25 mmol citric acid adduct at different temperatures (50–807C) and for varying lengths of time (15–240 min). Tab. 1 shows that: 1-The carboxyl content and the reaction efficiency percent increase on increasing the reaction temperature. 2-The increase in the carboxyl content and reaction efficiency percent depend on the reaction duration.

3.1.3 Reaction temperature and time

3-At the same temperature both the carboxyl content and the reaction efficiency percent increase by increasing the reaction time until they reach maximum values and then decrease.

Tab. 1 shows the effect of temperature and duration on the extent of the reaction occurring between starch and citric acid adduct in presence of 1.06 mmol NaOH. The

4-Maximum values of the carboxyl content depend upon the temperature; 54, 68.4, 180 and 145 meq / 100 g sample could be achieved at 50, 60, 70 and 807C, respectively.

Tab. 1. Effect of temperature and duration on the carboxyl content and percent of reaction efficiency (R.E.%). Duration [min]

15 30 60 120 180 240

Temperature [7C] 50

60

70

80

Carboxyl [meq/ 100 g sample]

R.E. [%]


R.E. [%]


R.E. [%]


R.E. [%]

12.4 23.6 40.0 54.0 38.0 17.8

3.8 7.2 12.2 16.4 11.6 5.4

33.6 42.7 55.6 68.4 56.9 38.6

10.2 13.0 16.9 20.8 17.3 11.7

58.0 83.3 124.5 180.0 161.0 130.6

17.6 25.3 37.8 54.7 48.9 38.7

113.0 129.7 145.0 131.0 2 2

34.4 39.4 44.1 40.0 2 2

Starch, 11 mmol; citric acid adduct, 8.25 mmol; sodium hydroxide, 1.06 mmol.



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5-The maximum values of the reaction efficiency percent depend upon the temperature: 16.4, 20.8, 54.7 and 44.1% were obtained at 50, 60, 70 and 807C, respectively.

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3.3 Evaluation of the anionic starch 3.3.1 Evaluation as flocculant 3.3.1.1 Effect of flocculant dose

6-Maximum values of the carboxyl content and reaction efficiency percent were achieved at 120 min in all samples at different temperatures except that at 807C they were obtained at 60 min. 7-The highest values of the carboxyl content and reaction efficiency percent were 180 meq/ 100 g sample and 54.7%, respectively and were achieved at 707C and 120 min.

3.2 Characterization of the anionic starch 3.2.1 Infrared spectroscopy Native starch and anionic starch sample having a carboxyl content 180 meq/100 g sample were characterized by infrared (IR) spectroscopy (Fig. 3). The carbonyl absorption of the anionic starch sample appears as carboxyl (I) at 1675.4 cm21 and (II) at 1574.4 cm21. These peaks can not be found in the spectrum of the native starch sample.

3.2.2 X-ray diffraction studies Starch granules are semicrystalline in nature. The crystalline areas comprise 20– 25% of the total volume of the starch granule [22]. The wide-angle X-ray diffraction pattern of native granular maize starch (Fig. 4a) comprises four more or less sharp peaks with low counts merely between 2y values of 10– 307, which on drying and modification with the citric acid adduct appear to have been compressed into a broad smoothened peak; i.e. the crystallinity of native granular starch has been altered after modification (Fig. 4b). It can therefore, be inferred that along with the amorphous region, the crystalline region of the granular starch is also involved in modification of starch.

Flocculation was carried out using the starch derivative based on H3-starch at various doses (ppm). This starch derivative had an acid content of 180 meq/100 g sample. Fig. 6 shows the effect of flocculant dose on the transmission % and weight removal % of the filtrate after flocculation at pH 6 and pH 8. It is seen that: (a) the transmission% and weight removal% increase with increasing the flocculant dose to reach a maximum value and then decreases. This phenomenon has been observed by several authors [18, 23, 24]; the flocculant dose giving rise to maximum transmission % and weight removal % value is considered as optimal dose value; (b) this phenomenon occurs at pH 6 and pH 8. The occurrence of an optimal dose value may be explained as follows: The ferric laurate suspension has an anionic demand of 2 and 1.6 for samples prepared at pH 6 and pH 8, respectively [25]. Addition of the anionic starch to the ferric laurate suspension leads to attraction between the suspended particles bearing positive charge and the acidic groups of the anionic starch. The starch derivative molecules attached to solid particles still have free active centers that can be adsorbed on the remaining free surface of other particles. This process results in “bridging” between solid particles and in the consequent formation of large flocs having a three-dimensional network structure. Beside bridging, the addition of the flocculant charge neutralization increases to reach zeta potential value of zero at which maximum flocculation occurs [26] (optimal dose value). After this situation, higher flocculant doses may impart an electric negative charge to suspended particles high enough to cause mutual repulsion. In addition, at higher doses the polymer covers most of the observed sites on each particle and bridging becomes negligible. The overall result is redispersion of the flocs because of the electrostatic repulsion.

3.2.3 Scanning electron microscopy

3.3.1.2 Effect of reaction medium (pH)

The granular structure of maize starch with spherulites in the size range between 0.1–10 mm is clearly shown in the micrograph (Fig. 5a), whereas a change in the shape of the starch granules after modification with citric acid adduct is seen in Fig. 5b. Individual starch granules have combined during the modification process.

Flocculation of ferric laurate suspension prepared at pH 6 and pH 8 was carried out by using anionic starch based on H3-starch bearing carboxyl groups (acid content 180 meq /100 g sample). Tab. 2 shows the transmission % and weight removal % of flocculated samples at the optimal flocculant doses. The data show that:



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Fig. 3a. IR spectrum of native starch. Fig. 3b. IR spectrum of modified starch (acid content is 180 meq/100 g sample).

(1) The optimal dose value depends on the pH of the flocculation medium and is higher at pH 6 than at pH 8. This is due to: (a) the higher anionic demand of ferric laurate at pH 6 than at pH 8; (b) The degree of

ionization of weak acids in the sodium salt is higher than that of the acid itself; (c) The molar extension of anionic flocculants at pH 8 is higher than at pH 6 (cf. Tab. 3).



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Fig. 4a. X-ray diffraction pattern of native starch. Fig. 4b. X-ray diffraction pattern of modified starch (acid content is 180 meq/100 g sample).

(2) The transmission % and the weight removal % obtained on using the optimal flocculant doses depend on the pH, being higher at pH 8 is higher than at pH 6.

(3) Flocculation at pH 8 consumes less flocculant than at pH 6 and gives higher transmission % and weight removal % values. This means that the flocculation efficiency of an anionic flocculant is higher at pH 8 than at pH 6.



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Fig. 5. Scanning electronic micrographs of (a) native starch and (b) modified starch (acid content is 180 meq/100 g sample).

Fig. 6. Effect of dose on the transmission % and weight removal % D Transmission % at pH 8 – m weight removal % at pH 8 h Transmission % at pH 6 – n weight removal % at pH 6 H3-Starch, 11 mmol; citric acid adduct, 8.25 mmol; sodium hydroxide, 1.06 mmol at 707C for 120 min. Acid content of all flocculants is 180 meq/100 g sample.

Tab. 2. Effect of pH value of ferric laurate solution on the transmission% and weight removal% obtained at optimal flocculant dose (ppm). pH

Optimal dose Weight removal [%] Transmission [%]

8

6

40 83 91

63 77 86

3.3.1.3 Effect of flocculant molar mass Flocculation of ferric laurate was carried out using various anionic starches based on native and hydrolyzed maize starches (acid content 180 meq /100 g sample). Tab. 4 shows the transmission % and weight removal % at the optimal dose of different samples. The data show that: (1) The optimal dose values decrease with decreasing molar mass of the flocculant and follow the order: native starch . H1-starch . H2-starch . H3-starch

Starch, 11 mmol; citric acid adduct, 8.25 mmol; sodium hydroxide,1.06 mmol at 707C for 120 min. Acid content of all flocculants is 180 meq/100 g sample.

(2) The transmission % and weight removal % increase when the molar mass of the flocculant decreases.



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Tab. 3. Intrinsic viscosity of the flocculant at pH 6 and pH 8. pH

Intrinsic viscosity

6 8

0.62 0.92

Tab. 5. Effect of acid content ([meq/100 [g] sample]) of the anionic flocculants based on H3-starch on the optimal dose D (ppm), transmission % (T) and weight removal % (W) of flocculated ferric laurate. Acid content [meq/100 g sample]

Tab. 4. Effect of molar mass of different flocculants on the optimal dose D (in ppm), weight removal % (W) and transmission % (T) at pH 8. Type of starch

D

W

T

Native starch H1-starch H2-starch H3-starch

65 49 44 40

46 69 73 83

53 76 80 91

Acid content is 180 meq /100 g sample. H1-, H2-and H3-starch have intrinsic viscosity [Z] values of 1.6, 0.8 and 0.6, respectively.

(3) Flocculation based on native starch required the highest flocculant dose and gave lowest transmission % and weight removal % while that based on H3-starch required the lowest flocculant dose and gave the highest transmission % and weight removal %, and those based on H1-starch and H2-starch lie in between.

399

60 100 140 160 180

pH 6

pH 8

D

T

W

D

T

W

94 82 72 67 63

64 72 79 82 86

55 63 70 73 77

45 44.5 43 41.5 40

69 76 83 85 91

64 70 76 78 83

Tab. 6. Removal (%) of anionic starch for different metal cations. Cation

Anion SO4

Mg21 Co21 Cu21 Hg21

2-

20.7 35.5 43.8 50.9

Cl2

2

26.5 37.0 50.1 61.3

31.5 42.9 60.0 73.3

OAC

Acid content of all samples is 180 meq/100 g sample.

(4) Thus, the highest flocculation efficiency can be obtained with flocculants based on H3-starch [18, 24].

their solutions. The cations were applied as acetate, chloride and sulfate salts. The obtained results are listed in Tab. 6.

3.3.1.4 Effect of acid content

It is seen from Tab. 6 that:

Flocculation of ferric laurate was carried out by using the anionic flocculant based on H3-starch with various acid contents at pH 6 and pH 8. Tab. 5 shows the optimal dose, transmission % and weight removal % of various samples. The data show that: (1) The optimal dose of flocculation decreases on increasing the flocculant acid content and depends on the pH. The optimal dose value is higher at pH 6 than at pH 8. (2) The transmission % of the flocculated solution and the weight removal% increase on increasing the flocculant acid content [18].

1-The metal removal by the anionic starch follows the order Hg21 . Cu21. Co21. Mg21 This is in accordance with the Irving-Williams series [27]. The difference in removal % of these different cations can be attributed to the equilibrium occurring during the exchange process, the different ability of these cations to form covalent bonds with the anionic groups of the exchanger and the ionic potentials of these cations. 2-The removal % depends on the gegen ions of the used cations and follow the order:

3.3.2 Evaluation of anionic starch as cation exchanger

acetate . chloride . sulfate.

Anionic starches bearing carboxyl groups (acid content 180 meq /100 g sample) were used for removing different heavy metal ions (Mg21, Co21, Cu21, Hg21) from

The difference is attributed to the difference in the ionization values (pKa) of these anions and its effect on the formed salt on the exchanger.



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4 Conclusions Starch-2-hydroxy-3-chloropropylcitric acid can be prepared using the dry process. The preparation is affected by the catalyst and citric acid adduct concentration, reaction time and temperature. The best conditions to prepare the anionic starch are a citric acid adduct: starch molar ratio 0.75:1, sodium hydroxide: citric acid adduct molar ratio 1:8, at 707C for 120 min to obtain an anionic starch with carboxyl content 180 meq /100 g sample. The modified starch was characterized by IR spectroscopy, X-ray diffraction and scanning electronic microscopy. The anionic starch derivatives act flocculants. The best result was obtained using hydrolyzed maize starch (H3starch) which has an acid content of 180 meq /100 g sample at pH 8. The transmission % was 91%, and weight removal 83% of the filtrate after flocculation at a dose of 40 ppm. The anionic starch samples were chelated with different metal cations such as Cu21, Co21, Mg21, and Hg21 salts in the form of acetate, chloride and sulfate salts. The metal removals% for the anionic starch follow the order Hg21 . Cu21. Co21. Mg21 The removal % depends on the gegen ions of the used cations and follows the order: acetate . chloride . sulfate

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