study on scale inhibition performances and interaction mechanism of modified collagen

6
Study on scale inhibition performances and interaction mechanism of modied collagen Xihuai Qiang, Zuhan Sheng , Hui Zhang College of Resource & Environment, Shaanxi University of Science and Technology, Xi'an 710021, China HIGHLIGHTS A scale inhibitor was prepared by modied chrome shavings hydrolyzing collagen. The scale inhibitor had good ability on calcium carbonate scale inhibition. The calcite crystal growth was inhibited by the scale inhibition. abstract article info Article history: Received 5 May 2012 Received in revised form 2 October 2012 Accepted 21 October 2012 Available online 29 November 2012 Keywords: Scale inhibitor Chrome shavings Inhibition mechanism Carboxyl modication The modied collagens with rich carboxyls were prepared by chemically modifying collagen, which was rst- ly extracted by hydrolyzing chrome shavings, using the multi-aldehyde acid compounds (MACs) as the mod- iers. Then those modied collagens and the blank collagen were used as scale inhibitors to implement scale inhibition tests. Scanning electron microscope (SEM) and XRD analyses were utilized to characterize mor- phology and crystal form of calcium carbonate scale. Results showed that the Ca 2+ concentration and pH were the main inuencing factors within the scale inhibition system, specically the scale inhibition efcien- cy would be reduced with the increase of Ca 2+ concentration and pH. Both the two types of scale inhibitors were more sensitive to the temperature, i.e., the scale inhibition rates were in a downward trend with aug- ment of the temperature. The crystal form of calcium carbonate could be completely distorted to form a vaterite crystal through the action of modied collagens, thus the scale inhibition effect is achieved. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Scale inhibitor is an important water treatment agent which is gener- ally added into the industrial circulating cooling water. Its main function is to keep the circulating cooling water system efciently transferring the heat and to ensure the normal operation of the industrial circulating cooling water. The common scale inhibitors mainly include inorganic phosphorus-containing polymer scale inhibitor, organic phosphate scale inhibitor, organic water-soluble polymer scale inhibitor, etc. [1]. Al- though the polymer scale inhibitor and phosphorus-containing scale in- hibitor are highly efcient as a scale inhibitor, they have some fatal aws such as difcult biodegradation in the water and eutrophication of the phosphorus-containing scale inhibitor. Therefore, the study that is to use natural products and their modied materials as biodegradable and eco-friendly scale inhibitors reaches great development [2,3]. In the traditional leather industry, 1 t of the raw leather can only be converted to 100 kg of nished leather while it generates more than 300 kg of solid wastes, most of which are chrome shaving. More impor- tantly, 80% of the chrome shavings are collagen-like protein [4,5]. China is one of the largest countries in leather production and each year the wasted chrome shavings can be up to 1.4 million t [6]. So the resource utilization of collagen-like protein in the wasted chrome shavings has always been a hot topic among the leather and chemistry industry, and environmental protection researchers. It is well known that there are some carboxyls and amino groups in the molecular chains of the protein and the carboxyls have the complexation function to Ca 2+ . However, the way to change the pendant amino groups into carboxyls via carboxylation modication will lead a certain amount of carboxyl groups into the protein molecular chains. In the present experiment, we have used the multi-aldehyde acid compounds (MACs) as the mod- iers and modied the collagen that was extracted by hydrolyzing the chrome shavings, in which way we have obtained the modied colla- gens with rich carboxyls. To study the scale inhibition of the modied collagen is to explore a feasible and reasonable methodology for the re- source utilization of collagen-containing solid wastes (chrome shav- ings) in leather production and the development of a new efcient scale inhibitor. 2. Experimental 2.1. Instruments and agents The instruments used in the present research included a precision force-increasing electric mixer, an RE-2000B rotary evaporator, a precisive pHs-3C, an S-4800 led emission scanning electron microscope Desalination 309 (2013) 237242 Corresponding author. Tel.: +86 29 86168675; fax: +86 29 86168291. E-mail addresses: [email protected] (X. Qiang), [email protected] (Z. Sheng). 0011-9164/$ see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.10.025 Contents lists available at SciVerse ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal

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Page 1: Study on scale inhibition performances and interaction mechanism of modified collagen

Desalination 309 (2013) 237–242

Contents lists available at SciVerse ScienceDirect

Desalination

j ourna l homepage: www.e lsev ie r .com/ locate /desa l

Study on scale inhibition performances and interactionmechanismofmodified collagen

Xihuai Qiang, Zuhan Sheng ⁎, Hui ZhangCollege of Resource & Environment, Shaanxi University of Science and Technology, Xi'an 710021, China

H I G H L I G H T S

► A scale inhibitor was prepared by modified chrome shavings hydrolyzing collagen.► The scale inhibitor had good ability on calcium carbonate scale inhibition.► The calcite crystal growth was inhibited by the scale inhibition.

⁎ Corresponding author. Tel.: +86 29 86168675; fax:E-mail addresses: [email protected] (X. Qiang), sh

0011-9164/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.desal.2012.10.025

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 May 2012Received in revised form 2 October 2012Accepted 21 October 2012Available online 29 November 2012

Keywords:Scale inhibitorChrome shavingsInhibition mechanismCarboxyl modification

The modified collagens with rich carboxyls were prepared by chemically modifying collagen, which was first-ly extracted by hydrolyzing chrome shavings, using the multi-aldehyde acid compounds (MACs) as the mod-ifiers. Then those modified collagens and the blank collagen were used as scale inhibitors to implement scaleinhibition tests. Scanning electron microscope (SEM) and XRD analyses were utilized to characterize mor-phology and crystal form of calcium carbonate scale. Results showed that the Ca2+ concentration and pHwere the main influencing factors within the scale inhibition system, specifically the scale inhibition efficien-cy would be reduced with the increase of Ca2+ concentration and pH. Both the two types of scale inhibitorswere more sensitive to the temperature, i.e., the scale inhibition rates were in a downward trend with aug-ment of the temperature. The crystal form of calcium carbonate could be completely distorted to form avaterite crystal through the action of modified collagens, thus the scale inhibition effect is achieved.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Scale inhibitor is an importantwater treatment agentwhich is gener-ally added into the industrial circulating cooling water. Its main functionis to keep the circulating cooling water system efficiently transferringthe heat and to ensure the normal operation of the industrial circulatingcooling water. The common scale inhibitors mainly include inorganicphosphorus-containing polymer scale inhibitor, organic phosphatescale inhibitor, organicwater-soluble polymer scale inhibitor, etc. [1]. Al-though the polymer scale inhibitor and phosphorus-containing scale in-hibitor are highly efficient as a scale inhibitor, they have some fatal flawssuch as difficult biodegradation in the water and eutrophication of thephosphorus-containing scale inhibitor. Therefore, the study that is touse natural products and their modified materials as biodegradableand eco-friendly scale inhibitors reaches great development [2,3].

In the traditional leather industry, 1 t of the raw leather can only beconverted to 100 kg of finished leather while it generates more than300 kg of solidwastes, most of which are chrome shaving. More impor-tantly, 80% of the chrome shavings are collagen-like protein [4,5]. Chinais one of the largest countries in leather production and each year thewasted chrome shavings can be up to 1.4 million t [6]. So the resource

+86 29 [email protected] (Z. Sheng).

rights reserved.

utilization of collagen-like protein in the wasted chrome shavings hasalways been a hot topic among the leather and chemistry industry,and environmental protection researchers. It is well known that thereare some carboxyls and amino groups in the molecular chains of theprotein and the carboxyls have the complexation function to Ca2+.However, the way to change the pendant amino groups into carboxylsvia carboxylation modification will lead a certain amount of carboxylgroups into the protein molecular chains. In the present experiment,we have used themulti-aldehyde acid compounds (MACs) as the mod-ifiers and modified the collagen that was extracted by hydrolyzing thechrome shavings, in which way we have obtained the modified colla-gens with rich carboxyls. To study the scale inhibition of the modifiedcollagen is to explore a feasible and reasonable methodology for the re-source utilization of collagen-containing solid wastes (chrome shav-ings) in leather production and the development of a new efficientscale inhibitor.

2. Experimental

2.1. Instruments and agents

The instruments used in the present research included a precisionforce-increasing electric mixer, an RE-2000B rotary evaporator, aprecisive pHs-3C, an S-4800filed emission scanning electronmicroscope

Page 2: Study on scale inhibition performances and interaction mechanism of modified collagen

238 X. Qiang et al. / Desalination 309 (2013) 237–242

(SEM), and an D/max2200PC X-ray diffractomer (XRD). The chemicalsused included analytical grade glyoxal andmelamine aswell as technicalgrade collagen powder extracted by hydrolyzing cowhide chrome shav-ings and glyoxalic acid (50%). Deionized water was used throughout thewhole experiment.

2.2. Methods

2.2.1. Preparation of chemicals

2.2.1.1. Preparation of multi-aldehyde acid compounds (MACs). Multi-aldehyde acid compounds (MACs) used in the present experimentwere synthesized according to the literature [7] and the specific proce-dures are described as follows in detail. First, add the melamine and anappropriate amount of deionizedwater into a three-neck flask equippedwith an electric mixer, a reflux condensing tube and a thermometer.Then heat the materials until the temperature reaches 50–60 °C. Atthis time add the sodium glyoxylate solution into the flask accordingto the ratio of a:1 (a=5, 4, 3, 2) between n (sodium glyoxylate) and n(melamine). Next heat the reaction system up to 70 °C and let it contin-ue reacting for 90 min. After a while, reduce the temperature to 40–50 °C and add the glyoxal aqueous solution based on the ratio of(6-a):1 between n (glyoxal):n (melamine). Warm the flask to 50 °C toreact for 90 min. In the end, adjust the solid content to 40%.

2.2.1.2. Preparation of modified collagens. Weigh 600 g of the preparedMAC solution and put it into the three-neck flask equipped with anelectric mixer, a reflux condensing tube and a thermometer. Thenadd in 15–25 g of collagen powders, adjust the pH to 7.5–8, and risethe temperature to 70 °C and keep the temperature for 4 h. At last,a condense yellow sticky solid is formed. Meanwhile, control thewater amounting to be 30%.

2.2.2. Test of scale inhibition performanceTaking the modified collagen as a scale inhibitor, we have

implemented the scale inhibition experiment referring to the nationalstandard (GB/T16632-2008) [8] of the People's Republic of China. Theprocess is as follows: add 250 mL of deionized water into a 500 mL vol-umetric flask, and 20 mL of standard calcium chloride solution with amass concentration of 16.7 g/L was added through a burette. A solutionof scale inhibitor sample of 0.5 g/L was added through a pipette andsubsequently shaken well. Then, a borax buffer solution with a massconcentration of 3.8 g/L was added and shaken well. 40 mL of standardsodium bicarbonate solution (shaken while adding) with a mass con-centration of 25.2 g/L was slowly added through a burette, and then di-luted to the scale with deionized water and shaken.

The test and blank solutions were placed in conical flasks, respec-tively, and then the two conical flasks were immersed in a thermo-static water bath (the level of the test solution should not be higherthan that of the water batch) of 80±1 °C to stay for 10 h at constanttemperature. The solutions were filtered with medium-speed quanti-tative filter papers after cooling to room temperature. The Ca2+ con-centration within the filtrate was titrated using titration, and the scaleinhibition rate of the scale inhibitor was calculated according to thefollowing formula. The impacts of those conditions on the scale inhi-bition rate were considered by different Ca2+ concentrations, exper-imental temperatures and pH values.

The scale inhibition performance η of a water treatment agentexpressed as a percentage was calculated according to formula (1):

η ¼ ρ1−ρ2ρ3−ρ2

� 100 ð1Þ

where ρ1, ρ2, ρ3 are the mass concentrations of calcium ion in the testsolution after the test using a water treatment agent and without use

of a water treatment agent as well as before the test, respectively, andtheir units are all mg/mL.

2.2.3. Sample collection of CaCO3 scaleAdd 120 mL of the 0.1 mol/L CaCl2 solution into a 500 mL beaker,

then 4 mL of 1 g/L scale inhibitor solution, and slowly pour 240 mL ofthe 0.1 mol/L NaHCO3 solution into the beaker to make the concen-tration of Ca2+ and HCO3

− in the solution to be 12 mmol/L and24 mmol/L, respectively. After fully and evenly mixing the solution,place it into thermostatic water of 30 °C for 10 h. Finally, take outthe scale and dry it in order to get the analyses subject of SEM andXRD.

3. Results and discussion

3.1. Structure of MACs and scale inhibition effect of modified collagens byMACs

According to the document [7], the conclusion is that the structure ofMACs is related to the molar ratios among the melamine, sodiumglyoxylate and glyoxal. Namely, we can obtain four types of MACs bycontrolling those molar ratios during the experiment. The structure di-agrams for the four kinds of MACs are shown in Fig. 1 as follows. InFig. 1, “a” means the molar ratios between the sodium glyoxylate andmelamine.

The chemical modification to the collagen was carried out throughthe reactions between the aldehyde groups in MACs and the aminogroups in collagen. Thus, the collagen molecular chain acquired a largenumber of carboxyl groups. The schematic diagram for the chemicalmodification principle of the collagen is shown in Scheme 1 as follows:

The modified collagens by four MACs and the blank collagen wereused as scale inhibitors to carry out scale inhibition tests and the resultsare displayed in Table 1. From Table 1, it could be seen that the scale in-hibition rates of themodified collagens were obviously higher than thatof the unmodified collagens, which is caused by the addition of the car-boxyls. It can chelate Ca2+ and reduce calcium carbonate scale accord-ingly. With the augment of carboxyls in MACs which were used tomodify collagens, the scale inhibition rate of the modified collagenwas gradually increased except the modified collagen by MAC 4. Thatis to say, scale inhibition rate of the modified collagen by MAC 3 wasthe highest and could reach 93%, but that of the modified collagen byMAC 4 lowered and reached 89%. This phenomenon can be explainedthat with the increase of carboxyls from MAC 1 to MAC 3, more andmore carboxyls are grafted onto the collagen chain and thus theirscale inhibition rates gradually rise. However, as far as MAC 4 isconcerned, there are up to 5 carboxyls but only one aldehyde groupon the MAC structure which will reduce the reaction between MAC 4and the collagen and the number of carboxyls grafted onto the collagenchain. Furthermore, due to steric hindrance among the molecules, for-mation of calcium scale with perforated mesh structure [9] is unfavor-able (Fig. 2).

3.2. Effects of scale inhibitor dosage and temperature

The experimentswere in order to study the patterns of how temper-ature and inhibitor dosage affect the scale inhibition performancesunder the condition of taking the unmodified collagen and the collagenmodified by MAC 3 as the scale inhibitors, and setting the pH of thewater sample to 6–6.5 and the concentration of Ca2+ to 250 mg/L. Re-sults are separately shown in Figs. 3 and 4. It could be seen that boththe scale inhibition effects of the two materials were gradually de-creased with the increase of temperature. The scale inhibition rate ofthe unmodified collagen to the calcium carbonate in aqueous solutionwas relatively low, and the maximum scale inhibition rate was notmore than 35%. It is well known that there is only 22.6% of thecarboxyl-containing amino acids per 100 amino acid residues within

Page 3: Study on scale inhibition performances and interaction mechanism of modified collagen

MAC 1 (a=2) MAC 2 (a=3)

MAC 3 (a=4) MAC 4 (a=5)

N

N

N

N

NN

CH CH

CH

CH

CH

CH

CHO

OH

NaOOC

OH

OH

CHO

CHOHO

OH

NaOOC

OHOHC

N

N

N

N

NN

CH CH

CH

CH

CH

CH

CHO

OH

NaOOC

OH

OH

CHO

COONaHO

OH

NaOOC

OHOHC

N

N

N

N

NN

CH CH

CH

CH

CH

CH

COONa

OH

NaOOC

OH

OH

CHO

COONaHO

OH

NaOOC

OHOHC

N

N

N

N

NN

CH CH

CH

CH

CH

CH

COONa

OH

NaOOC

OH

OH

COONa

COONaHO

OH

NaOOC

OHOHC

Fig. 1. Structure of MACs.

239X. Qiang et al. / Desalination 309 (2013) 237–242

the rawmaterial of protein powder [10]. So the lack of the carboxyls inthe side chain of the protein molecule directly caused the low scale in-hibition rate. When the inhibitor dosage of the unmodified collagen ex-ceeds the best concentration of 25 mg/L, the scale inhibition rate atdifferent temperature lowers, which is mainly the reason that underthe condition of the high concentration, the flocculation of the largemolecule polypeptide dominates and it engenders the flocculation ofcalcium ions to some extent. Thus, the scale inhibition effect will be re-duced. And this flocculation effect of collagen andmodified collagen hasalso been found by Sha Zhang [11].

However, the scale inhibition efficiency of the collagen modifiedby MAC 3 to the calcium carbonate was significantly increased, andthe maximum inhibition rate could reach to 93%. It is probable thatfive carboxyl groups can be introduced into the molecular chain pen-dant groups of modified collagen by MAC 3 and this should greatlycombine more Ca2+ in aqueous solution. The scale inhibition rate isthus distinctly increased. When the inhibitor dosage was more thanthe optimum concentration of 35 mg/L, the scale inhibition rates atdifferent temperatures were in a downward trend. The reason forthis phenomenon is the same as the unmodified collagen, and it isthe flocculation effect.

P NH2

OHC A COONa+a

Scheme 1. Synthesis schematic diagram of collagen based on scale inhibitor P indicates the dlecular skeleton of multi-aldehyde acid.

From Figs. 3 and 4, it was also displayed that temperature has a bigeffect onmodified collagen and unmodified collagen, specifically the in-hibition rates of modified collagen and unmodified collagen were re-spectively reduced to some extent with the rise of temperature.Particularly, the inhibition rate of the modified collagen was reducedby a big margin when the temperature was higher than 80 °C. Thehighest inhibition rate of the modified collagen was 80% and it's onlywhen the temperature was higher than 80 °C. The inhibition perfor-mancewas reduced sharply. In conclusion, the effect of the temperatureon a scale inhibitor is more sensitive.

3.3. Effect of pH

The experiments studied the influencing rules of pH value on thescale inhibition rates by taking the collagen and the modified collagenby MAC 3 as scale inhibitors under the condition that the Ca2+ con-centration was 250 mg/L. Please refer to the results in Fig. 4.

Fig. 4 shows that both the scale inhibition rates of the two scale in-hibitors were in a linear decreasing trend with the raise of pH. Themain reason is that with increase of pH, the OH− concentration in-creases and the reaction between OH− and HCO3

− speeds up, which

P N CH A COONa a

iagram for the molecular skeleton of collagen; and A indicates the diagram for the mo-

Page 4: Study on scale inhibition performances and interaction mechanism of modified collagen

0 10 20 30 40 50

10

20

30

40

50

60

70

80

90

100

Sca

le in

hibi

tion

(%)

Concentration of modified collagen (mg/L)

60˚C 70˚C 80˚C 90˚C

Fig. 3. Effects of temperatures and concentrations on scale inhibition efficiency of mod-ified collagen by MAC 3.

Table 1Scale inhibition rate of the modified collagens by MACs and the blank collagen⁎.

Scaleinhibitors

Modifiedcollagen byMAC 1

Modifiedcollagen byMAC 2

Modifiedcollagen byMAC 3

Modifiedcollagen byMAC 4

Blankcollagen

Scaleinhibitionrate (%)

61 78 93 89 35

⁎: Experimental temperature was 60 °C; the inhibitors condense at 35 mg/L, pH 6–6.5,Ca2+ concentration of 250 mg/L.

240 X. Qiang et al. / Desalination 309 (2013) 237–242

should lead to the decrease of HCO3− concentration and the increase

of CO32− concentration, thus, calcium carbonate precipitation forms

[12] and the scale inhibition rates linearly decline.In view of structures of the collagen protein and modified collagen

protein, their major functional groups are carboxyl groups. The scaleinhibition mechanism of carboxylic acid scale inhibitor is due to acomplex network structure formed between the carbonyl groupC_O and the\OH group from the adjacent molecule, which providesa place for the adsorption of inorganic dirt microcrystalline, and thusthe scale inhibition effect can be in effect.

When pH is relatively low, besides the C_O from the partiallyprotonated scale inhibitor containing carboxylic acid groups, theunprotonated \COOH can also react with the surface of insolublesalts embryos to enhance the scale inhibition effect [13]. While thepH is relatively high, the ionization degree of the carboxylic acidgroup will increase, only the C_O from the completely protonatedscale inhibitor containing carboxylic acid groups can form hydrogenbonds with the water molecules from the insoluble salts embryos sur-face. Finally, the scale inhibition performance is weakened.

3.4. Effect of Ca2+ concentration

When the feeding concentrations of the collagen and the collagenmodified by MAC 3 were 25 mg/L and 35 mg/L, and both the temper-atures and pH of the two scale inhibition systems were 60 °C and 6–6.5, respectively. The influencing rules of calcium ion concentrationon the scale inhibition performances using the collagen and the colla-gen modified by MAC 3 as scale inhibitors were conducted in this ex-periment and the results are shown in Table 2.

The experimental results of Table 2 showed that both the scale inhi-bition rates of the two agents gradually decreased with the increase ofCa2+ concentration. For the modified collagen material, the decreasetrend of scale inhibition rate was gentle when the Ca2+ concentrationwas more than 450 mg/L. However, the scale inhibition rate of

5 10 15 20 25 30

0

5

10

15

20

25

30

35

60˚C 70˚C 80˚C 90˚C

Sca

le in

hibi

tion

(%)

Concentration of protein (mg/L)

Fig. 2. Effects of temperatures and concentrations on scale inhibition efficiency of collagen.

unmodified collagen was always in a linear downward trend with thegradual increase of Ca2+ concentration. This phenomenon indicatesthat the Ca2+ adaptability of modified collagen is better than that ofthe unmodified collagen.

3.5. Characterization of calcium carbonate scale

In order to better analyze the impact of modified collagen on thegrowth of CaCO3 crystal, the experiment used unmodified collagenpowder and the modified collagen powder by MAC 3 as scale inhibi-tors, collected CaCO3 scale according to the experimental methodunder the optimum feeding concentration, and the collected CaCO3

crystal was characterized by SEM and XRD analyses. The results areshown in Figs. 5 and 6.

Fig. 5 showed that the CaCO3 crystal (a) generated in the blank sam-ple without any agent mainly comprised calcite, which was featuredwith clear cube geometric structure, smooth surface, regular shape,and compact structure. When the blank collagen was added, the gener-ated CaCO3 crystal obviously became irregular, specifically part of thecrystal had shown as a bundle aragonite structure. But its structurewas still compact, part of the surface was relatively smooth, and theedges and corners were also relatively clear. The surface of the crystalwas displayed rough from Fig. 5(c) after adding the modified collagen.

5 6 7 8 910

20

30

40

50

60

70

80

90

Scale inhibition of collagenScale inhibition of modified collagen

Sca

le in

hibi

tion

(%)

pH

Fig. 4. Effect of pHon the scale inhibition rates of collagen andmodified collagenbyMAC3.

Page 5: Study on scale inhibition performances and interaction mechanism of modified collagen

Table 2Effect of Ca2+ concentration on inhibition properties of collagen and modified collagen⁎.

Ca2+ concentration (mg/L) 150 250 350 450 550 650 750

Scale inhibition rate of collagen (%) 35 27 22 16 10 7 4Scale inhibition rate of modifiedcollagen (%)

94 85 75 54 50 48 45

⁎: The feeding concentrations of the collagen and the collagen modified by MAC 3 are25 mg/L and 35 mg/L respectively; both the temperatures of the two scale inhibitionsystems are 60 °C.

241X. Qiang et al. / Desalination 309 (2013) 237–242

However, the whole structure was relatively smooth, the crystal waspresented as in the form of vaterite as well as partial square bundle ara-gonite, its structure was looser than that in Fig. 5(a) and (b). The crystalsize was reduced dramatically compared to that of (a) and (b).

It is known that both solubility product and free energy of the bun-dle aragonite and the vaterite are higher than those of the calcite [14].So the crystalline generated is unstable and highly dispersive, and thesoft dirt is easy to dissolve and flows freely withwater. Thus the crystal-line and the soft dirt are difficult to attach to thewall of the beaker in theexperimental process, i.e. easy to wash away by the water. On basis ofthe analyses for SEM diagram and the crystal distortion theory, thefunctional group of carboxylic acid has a chelating ability to the metalions, which will interfere with the formation of inorganic scale crystal-lization and the growth of the crystalline cannot be in strict accordancewith the normal lattice, at last the generated crystal is irregular. In addi-tion, comparison between (b) and (c) showed that the lack of carboxyl-ic acid groupwas themain reason for the difference of crystal structure.

Fig. 6 showed that the diffraction peak strength of the calcite crys-tal 2θ formed within the blank sample without agents at 29.301° (thecharacteristic crystal face 104 of the calcite) and 48.420° (the charac-teristic crystal face 116 of the calcite) was the strongest, which illus-trated that the faces (104) and (116) were the major growth surfaces.This result is in accordance with the study results based on molecularmechanics method executed by H dicke et al. [15]. In addition, the

Fig. 5. Morphologies of the CaCO3 precipitation in the absence of an inhibitor

diffraction peaks at 35.880°, 39.320°, 43.079°, and 47.420° were rela-tively strong. The crystal 2θ formed after adding unmodified collagenalso had the diffraction peaks at 29.301°and 48.421°, but the peakstrength was greatly reduced, which indicates that the growth ofthe crystal faces 104 and 116 is mainly inhibited by the collagen.After adding the modified collagen by MAC 3, the diffraction peakstrength of the 2θ corresponding to the crystal faces 104 and 116was further weakened, meanwhile, the calcite characteristic diffrac-tion peaks of 2θ at 43.079° and 47.420° were simultaneously andgradually reduced, which indicates that the growth of calcite crystalcan be inhibited by unmodified collagen and modified collagen, andthe inhibition effect of modified collagen is stronger than that ofunmodified collagen. Fig. 6 also displayed that the 2θ in the XRD dia-gram, after adding unmodified collagen scale inhibitor, had the crys-tal face diffraction peaks of vaterite at 20.879°, 24.781°, 43.759°, and49.960°. Furthermore, the above diffraction peaks also could be seenin the XRD diagram after adding modified collagen, and the strengthof the diffraction peaks was stronger than that of unmodified colla-gen. This result accords with SEM results. The above analyses showthat some calcite crystals are changed into another CaCO3 structure,i.e., vaterite after adding unmodified collagen and modified collagen.Since the calcite is the most stable structure of calcium carbonate, theformed scale is often very hard and easy to deposit on the surface ofthe heat exchanger. Moreover, solubility product and free energy ofvaterite are higher than those of calcite, the unstable and dispersedcrystalline and soft dirt are easy to dissolve and flow with water,namely difficult to attach to the surface of the heat exchanger. As a re-sult, the impact on heat transfer can be prevented and the scale inhi-bition effect can be acquired.

4. Conclusion

(1) The modified collagens with rich carboxyls were prepared bychemically modifying the collagen, which was firstly extracted

(a), in the presence of unmodified protein (b) and modified protein (c).

Page 6: Study on scale inhibition performances and interaction mechanism of modified collagen

Fig. 6. XRD images of the CaCO3 crystal formed in the absence of an inhibitor (d), in the presence of unmodified protein (e) and modified protein (f).

242 X. Qiang et al. / Desalination 309 (2013) 237–242

by hydrolyzing chrome shavings, using the multi-aldehydeacid compounds (MACs) as the modifiers. The scale inhibitionrate of the modified collagen was significantly higher thanthat of the unmodified collagen, thus the modified collagen isa favorable scale inhibitor with application prospects and pro-vides a feasible and reasonable method for the resource utiliza-tion of waste collagen in leather production.

(2) In the scale inhibition system, Ca2+ concentration and pH werethe major influencing factors, specifically the scale inhibition ef-ficiency would be reduced with the increase of Ca2+ concentra-tion and pH. Themodified collagenhad a good adaptability to theCa2+, although the scale inhibition rate was higher under acidicconditions, in view of its corrosive effect on the equipment, theappropriate pH was 6–7 during the application process of thescale inhibitor.

(3) The optimum concentration for the modified collagen as scaleinhibitor and temperature were 35 mg/L and 60 °C, respective-ly, and the corresponding scale inhibition rate could reach to94%, which was 2.7 times of that of the unmodified collagen.

(4) The interaction mechanism of the modified collagen scale inhib-itor was mainly originated from the effect of lattice distortion.Specifically, some active sites on the crystal growth surfacewere occupied by this scale inhibitor and thus the crystal growthwas inhibited or the generated crystalwas porous, loose and easyto flow with water without depositing to form a hard scale.

Acknowledgments

This work was supported by the Foundation for Innovative ResearchTeam of Shaanxi University of Science &Technology (TD09-04) and theDoctoral Scientific Research Fund of Shaanxi University of Science&Technology (BJ09-15).

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