Water-Soluble Graft Copolymer Synthesized from ... - ACS Publications

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Industrial & Engineering Chemistry Research

A Water Soluble Graft Copolymer Synthesized from Collagenous Waste and PEG with Functional Carboxylic Chains: A Highly Efficient Adsorbent for Chromium(III) with Continuous Recycling and Molecular Docking Studies James Kanagaraj *1, Rames C. Panda 2 and Vijayan Sumathi 3 1

CSIR-CLRI, Adyar, Chennai-20, India

2

Chemical Engineering Division, CSIR-CLRI, Adyar, Chennai-20, India 3

Vellore Institute of Technology, Vellore, India *1 Corresponding author CSIR- Central Leather Research Institute, Adyar, Chennai-600020, India.

Ph: 044-24911386 (Off), 044-244530630 (Res), Fax: 91-044-24911589. Email: [email protected]

ACS Paragon Plus Environment


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ABSTRACT: Pragmatic way to zero emission is achieved by utilizing solid and liquid wastes completely by recycling and generating value added auxiliaries from chrome shaving wastes through process intensified unit operations. Graft copolymer was synthesized from chrome shaving wastes and Poly Ethylene Glycol which is used as adsorbent in the fresh chrome tanning process.

The copolymer was characterized by TGA, DSC,

FT-IR, particle size analyses. FT-IR results characterized confirming the presence of free functional carboxylic acid involved in chromium adsorption. Application of 6% of the copolymer in chrome tanning showed 95.95% and 98.90% optimal adsorption of chromium in the first cycle and second cycle respectively.

1

H NMR investigation

revealed peak at 8.384 ppm (6% copolymer) in the experimental sample confirming the participation of free functional carboxylic acid groups that play main role in adsorbing chromium. Molecular modeling showed increased adsorption of chromium and more participation of ligand-copolymer with collagen resulting in docking energy of 3.9 kcal/ mol through hydrogen bonding establishing the spatial arrangement of the active functional groups. A mathematical model for the prediction of concentration of chromium adsorbed by the copolymer has been proposed. SEM and SEM-EDX studies confirmed increased adsorption of Cr especially, 6.33 weight % of Cr containing atomic % is 2.03 in sample containing 6% copolymer. AFM studies from topography and deflection showed clear pictures of increased adsorption of chromium by copolymer indicating absence of chromium spots on the surface for the experimental samples. Key words: Graft co-polymer, adsorption of Cr(III), molecular modeling, AFM studies, SEM-EDX studies


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INTRODUCTION Though chromium is available in nature mostly as chromium(III) or chromium(VI) states, chromium(VI) exhibits toxicity 500 times more than chromium(III).

1

However,

chromium(III) is essential for human nutrition in the metabolism along with insulin. The reduction of Cr(VI) to Cr(III) has been demonstrated using bio-sorbent surface under acidic condition, higher raw material concentration and temperature.

2-6

Cr(III) is widely

used in tanning for conversion of putrescible skin into imputrescible leather. These processes show conventional adsorption of Cr(III) up to the levels of 60-70% while the remaining goes with effluent containing 2000-4000 ppm of Cr(III) which poses potential problems to environmental compliances. The process could be modified by improving the adsorption by employing high performance auxiliaries to improve the stabilization of Cr(III) with collagen. Continuous recycling of the effluent will also help in enhancing the performance of exhaustion.

7-11

In the entire process while 20% of the raw materials

gets converted into imputrescible leather, most of the matter goes along with solid wastes of which about 20% comes from the collagen bearing chrome shaving solid wastes.12 These wastes were disposed to landfills in earlier days. However, strict environmental norms have urged the industry to seek viable options to produce value added product for direct use or for application in further process as high performance augmenter to the auxiliaries. Direct application includes compounds of making amino acids,

13

fibrous sheets based on acrylate,14, 15 polymeric materials

16-18

, insulators or

building material and then animal feed etc while indirect use of these protein based wastes is used either to separate or to enhance adsorption of Cr(III) on the collagen matrix for stabilization of leather.19,

20

Literature reports that trials to hydrolyze these



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wastes were undertaken in preparation of wet blue leathers. Separation and recovery of Cr(III) requires costly technologies. Fleshing wastes were hydrolyzed and the hydrolyzates were copolymerized using acrylic acid for the application of enhancement of Cr(III) exhaustion and nano-particle based polymer was prepared from keratin hydrolyzate and acrylic ester for use in dyeing. 21-23 In a separate approach, the chrome shaving wastes were used as reducing agent for Cr(VI) in preparation of chrome(III) tanning salt.24 These protein based wastes were also used after several hydrolysis for the preparation of tanning agent to provide a zero discharge in chrome containing leather waste. In the present investigation, protein based chromium waste has been dechromed with oxalic acid followed by alkaline hydrolysis to get collagen hydrolysate (CH). This intermediate CH was polymerized with poly ethylene glycol (PEG) to get low molecular weight based copolymer. PEG exhibits important properties as good binder, high permeability and retention factor, good osmotic pressure, hydrophilic properties. Also, it is used as a preservative for many substances. These attributes have motivated the authors to select PEG as one of the monomer for the preparation of copolymer. The resultant co-polymer product was characterized for particle size, TGA, DSC, FT-IR, 1

HNMR for confirming the presence of active functional groups. This copolymer was

tested as adsorbent of Cr(III) in the tanning process and the resultant tanning agent was recycled with the fresh feed in the next batch of chrome tanning.


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EXPERIMENTAL SECTION Materials Chrome shaving wastes (collagen wastes) were collected from the tannery division of CLRI. Basic chromium sulphates (BCS), PEG, acetone, potassium persulfate were purchased from Sigma Aldrich. Pelt (skin ready for tanning) ready was procured for adsorption studies during chrome tanning from local vendor. Synthesis of water soluble graft copolymer Graft co-polymer was prepared using monomers of CH and PEG. 250 mL of distilled water and was taken in a three necked round bottom reaction flask attached to magnetic heating system at 90°C with constant stirring. Then 20 g of CH was added and stirred for 60 min with heating to make homogenous mixture. 40 g of PEG (was dissolved in sufficient amount of methanol) was added in drops through one of the necks of the flask while initiator, potassium persulfate weighing 1.5 g dissolved in 50 mL of water, was also added in installments through the other neck of the flask. The reaction was allowed to proceed for 3 h with constant heating at 85-90oC with constant stirring. The pH of the resultant product was recorded as 2.5 which were adjusted further to pH of 4 with aqueous solution of sodium bi carbonate. Finally, the co-polymer product was cooled (using desiccators) at room temperature and was stored. The characteristic feature of the product was analyzed for various parameters using standard methods. Determination of percent grafting and grafting efficiency A number of the experiments were carried out to optimize the polymeric conditions to get the desired results. Percent grafting (% PG) and grafting efficiency (% GE), were



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determined after completing the polymerization, the product was extracted using Soxhelt apparatus with methanol to remove the homopolymer. When the extraction time was complete, the sample was carefully transferred into an evaporating dish and the sample was weighed. The percent PG and GE were calculated according to the formulas given in eqs 1 and 2 by adopting the standard procedure. Percentage grafting (PG) =

Grafting efficiency (GE)=

weight of the graft polymer X 100 weight of the back bone (polymer)

weight of the grafted copolymer X 100 weight of the total polymer (homo + grafted)

(1)

(2)

Characterization of graft copolymer Differential scanning calorimetry (DSC) was performed to determine the transition temperatures like glass transition temperature (Tg), melting temperature (Tm), and crystallization temperature (Tc) of the graft copolymers. DSC of the samples was carried out using Q2000 TA Instruments with ramp 15 °C/min from room temperature to 300 °C in a nitrogen atmosphere. Thermogravimetric analysis (TGA) of the sample was carried out using a Q50 TA Instruments, ramp 15 °C/min from room temperature to 600 °C in a nitrogen atmosphere. Weight loss of these materials as a function of temperature was recorded using this study. The viscosity of the copolymer was done in Brookfield DV-E viscometer with different spindle nos. (nos. 07, 06, 05, 04, and 03) and at different speed (20, 30, 50, 60, and 100 rpm). The particle size and zeta potential measurements were performed on copolymer using Malvern zeta sizer model 1000HS/3000HS, UK. Particle analysis was performed at a fixed scattering angle of 90°. The molecular weight of the copolymer isolated from dispersion using methanol as precipitating solvent was found. The GPC instrument is fitted with ultrastyragel columns


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(pore size 103 -105 Å, also supplied by JASCO International Co.Ltd. Japan) and refractive index detector. Tetrahydrofuran solvent was used as an eluting solvent and the flow rate was maintained at 1mL min-1. The % solid level was found out by conventional evaporation method. Continuous recycling of chrome tanning process Two recycling experiments were carried out for the present investigation. The copolymer prepared from the chrome shaving wastes had been applied at various levels such as 3, 6 and 9% in the tanning process and the % absorption of chromium was studied. The skins after pickling process (with pH about 3.0) are considered for experiments and are subjected to this tanning. The skins were treated with 8% of BCS (chromium (III) tanning agent) in a rotating drum/ vessel for a period of 2 h and were followed by basification using sodium formate and bicarbonate where the pH of the leather was adjusted to 4.0. The spent liquors were collected. Then copolymer at the level of 3, 6 and 9% was added with the chrome-treated leather with constant stirring in the rotating vessel for a period of 1 h for complete reaction and penetration. The collected spent liquors were recharged and the process was continued for another hour. The spent liquors were collected and analyzed for the % adsorption of chromium. After adsorption of chromium from the bath, the spent liquor was collected again and precipitated by using magnesium oxide (MgO) with the ratio of 2:1 (MgO:Cr) and redissolved by using dil. sulfuric acid to get the regenerated form of chromium. The regenerated chrome liquor from the chrome residue/sludge was reused in the fresh batch (2nd cycle) of chrome tanning along with the required amount of BCS and the pelt was tanned. Likewise, another batch of chrome tanning was carried out using the



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copolymer and the exhaustion was studied. Spent liquor is collected and chromium is recovered by above method while the supernatant is discharged to effluent.

The

schematic flow chart for the 1st and 2nd cycle of chrome tanning with copolymer is presented in Scheme 1. The docking of copolymer ligand with collagen by computational study Collagen structure (treated with Cr) was modeled by using gencollagen package. 25 The basic unit of the co-polymer was taken for the docking study. The structure of ligand was built using chemsketch and the obtained geometry was used for docking with collagen. The docking of ligand with collagen was performed as blind dockings (blind docking refers to the use of a grid box which is large enough to encompass any possible ligand receptor complex) using Auto Dock Vina program.

26

The lowest binding

energy docked complex obtained was taken for the further study. The obtained data from docking was examined by PYMOL software.

27

Methods for estimation of chromium, 1H NMR spectra with water suppression using watergate sequence, FT-IR analysis, mathematical modeling, SEM, SEM- EDX studies, 29

AFM study 30 have been adopted using the standard procedures.

RESULTS AND DISCUSSION Characterization of the copolymer The

co-polymer

synthesized

from

the

collagen-chrome

shaving

waste

was

characterized before it was used in the study. The results are presented in Table S-1 and Figure S-1(a-d). The particle size of the co-polymer was found to be 310 nm, relative viscosity of 0.8872 cp, poly dispersity index of 0.555 with solid level of 5.05 % and molecular weight of 5.10 x105 D. This is a relatively low molecular weight copolymer


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designed to adsorb chromium from the water medium.

This particle size of the co-

polymer with very well-water-miscible nature adhering to the above characteristics are very helpful in using this as adsorbent in the tanning. The ratio of number average and weight average molecular weight of the polymer termed as polydispersity index has been well established to serve as useful parameter in drawing information on molecular weight distribution. The copolymer developed in the present work shows narrow polydispersity index at 0.555 with viscosity of 0.8872 cp.

The generation of low

molecular weight copolymer with narrow polydispersity index has been achieved through CH and PEG polymerization reaction. It is significant to note that in absence of CH it was not possible to perform micro-emulsion polymerization reaction with the PEG monomer. DSC was performed to determine the thermal transition temperatures like glass transition temperature (Tg), melting temperature (Tm), and crystallization temperature (Tc) of the copolymer. The (Figure S-1b) shows peak at 158.62o C representing glass transition temperature of the co-polymer. It is attributed to the PEG residue which is close to boiling point of PEG. TGA curve shows three stages of decomposition initial stage at 374.62°C, intermediate stage at 458.02°C and final stage at 748.57°C (Figure S-1a). This may be due to the lower molecular weight of the co-polymer that is required for the adsorbent in effective diffusion through pores of the collagen matrix. Similar characteristic and adsorption features were also reported by other researcher.31 The measurement of zeta potential is critical in understanding the surface electrical charge characteristics of copolymer (Figure S-1c). The electrical charge properties control the interactions between chromium compounds and therefore determine the



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overall behavior of a sample adsorption. The zeta potential for each particle in the media is the measure of the electrical potential at the slip plane between the copolymer particle and the bulk solution containing chromium. It is the ‘zeta potential’, that is the main controlling factor of the stability of sample particles in liquid media, i.e. to what degree adsorption/aggregation will occur between the copolymer and chromium over time. The zeta potential of copolymer sample is estimated to be at –0.0403 mV. The negative value of zeta potential of the sample demonstrates coverage of the copolymer particle indicating weak anionic nature of the copolymer particle. The zeta potential shows almost similar charge characteristics of collagen (with iso-electric point of pH of 4.7). The residual surface charge of the collagen counter-produce suits well for the adsorption of copolymer. This copolymer adsorbs chromium from aqueous environment having similar pH conditions subsequently binding with collagen matrix. Mechanism and hypothesis of grafting of PEG onto CH The mechanism behind grafting PEG onto CH is due to formation of esters by combining free carboxylic acid groups of collagen hydrolyzate and OH groups from PEG. These esters play secondary role in masking and stabilizing Cr complexes for improved adsorption of chromium and primarily with free functional carboxylic acid for the adsorption process. Besides esters, free function carboxylic acids generated from unreacted polymers (CH) directly involve in improving adsorption through hydrogen bonding. Reaction scheme of graft co-polymer has been provided as below (Scheme 1). The carboxylic acid groups present in the collagen hydrolysate reacts with PEG to form ester groups that play important role in adsorbing Cr (III).


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Scheme 1. Grafting of PEG onto CH It was found from the grafting experiments that the degree of grafting to be approximately 60.0%. Number of functional acid groups of CH has been engaged / occupied in formation of esters while few are remaining unoccupied (Table S-2). Continuous recycling of chrome tanning using synthesized co-polymer Chrome shaving wastes are collected and are hydrolyzed after sufficient de-chroming and co-polymerized with PEG to get the copolymer which is used in chrome tanning experiment as an aid / adsorbent. Two batches of chrome tanning with continuous recycling have been carried out where copolymer at various levels 3, 6, and 9% were treated with leather after addition of BCS in the 1st batch. After the adsorption of chromium, collected liquor was analyzed for chrome content and is presented in Table 1. The Table shows adsorption of chromium for different cases of 3, 6 and 9% of addition of co-polymers. % of Cr2O3 present in the spent liquor for different sets of experiments (with addition of co-polymer) is provided in the Table 1. It can be found that the % of Cr2O3 for the addition of 3, 6 and 9% of co-polymer gives 2.597 g/l, 1.1822 g/L and 1.67 g/L respectively. % adsorption in these cases resulted in 93.58, 95.95 and 95.88% showing a maximum at 6% level of addition of co-polymer. Thus addition of 6% of co-polymer is enough to carryout for maximum adsorption of chromium from the bath.



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Remaining amount of chromium present in the bath is recovered by precipitating with MgO followed by re-generation of chromium using dil. sulphuric acid, and is reused in the 2nd batch of chrome tanning experiment with addition of required amount of fresh BCS. Results obtained from 2nd batch are also given in Table 1 where the % adsorption was found to be 96.85, 98.90 and 98.82% in the spent bath for the addition of 3, 6 and 9% of co-polymer respectively. Analysis showed that 6% of addition of copolymer was sufficient for maximum adsorption of chromium. The reason behind this increase in adsorption level is due to presence of esters and free functional groups of carboxylic acid sites which was also confirmed by 1HNMR studies. The reactive functional groups responsible for improved adsorption of chromium were investigated by earlier researchers.

32-36

It is to be noticed that solid waste /sludge that are generated from

chrome tanning is fully utilized / recycled in the next batch leaving no chromium in the effluent. The solid waste in the form of chrome shavings have been utilized for making the co-polymer. This method paves the way for zero emission in the tanning process. In the present investigation, the grafted CH–PEG co-polymer that is employed for tanning process after the fixation of Cr (III) plays a major role in inviting / attracting more amount of Cr(III) from aqueous medium to the collage matrix and thereby more adsorption takes place finally resulting in higher exhaustion of chromium in the tanning process. The improved adsorption leading to high exhaustion takes place in three possible reactions. Mechanism of improved chrome adsorption The main important reason for higher adsorption of chromium is that free functional carboxylic groups present in the copolymer helps for the adsorption of Cr. The free


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carboxylic acids of copolymer may form hydrogen bonding at multipoint in providing additional adsorption and exhaustion to the complex. The presence of free functional carboxylic groups were confirmed by 1H NMR studies and discussed in the later sections. The other reason behind this improved adsorption may be due to the fact that OH groups of PEG that forms H-bonding with carboxyl groups of collagen. In addition to that a physiochemical property of ester is responsible for masking the Cr complexes that indirectly helps in stabilizing Cr complex in adsorption process. Masking reduces the potency of the chromium thereby increases the reactivity of chromium with the collagen matrix. Masking also favors for thorough/uniform distribution of chrome complexes and to satisfy all the reactive groups of collagen, thereby higher adsorption of chrome is achieved. Different materials were employed to enhance adsorption of chromium in tanning.

34-38

Proteinacious materials containing reactive groups, such as, carboxylic acid, amino groups are effective in cross linking with chromium at multipoint imparting additional stability.

Use of nano particle prepared from fleshing hydrolyzate,

fleshing hydrolyzate polymer, nano-composite

37

,

22

keratin hydrolyzate,

oxazolidine derivatives,

38

33, 35

21

acrylic acid-

akovite−alumino silicate

cationic polymer, silk hydrolysate,

39

struvite from phosphorus recovery can improve the adsorption of Cr (III) from aqueous media. 40 Similar type of copolymer using poly vinyl alcohol and CH has been developed for the enhancement of chromium.

41

Biosorption of chromium from effluent using

Bacillus sp. have been attempted for reduction of pollution. 3 Researchers in above said works confirmed participation of reactive either carboxylic acid, amino or aldehyde groups favor the improvement of adsorption of chromium in the experiment. They have



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proposed the mechanism of adsorbing chromium with the co-polymer and then binding to the collagen matrix has been discussed.

42

Applications of similar type of polymers

and copolymers for the various applications have been discussed which are very helpful in designing the characteristics features of the adsorbent for the particular type of work. 43-46

Molecular modeling for docking of copolymer ligand with collagen The docking of copolymer ligand and collagen was carried out for understanding the mechanism of improved adsorption of chromium by the copolymer. Chemsketch was used for building the ligand copolymer.

25-27

The docking of copolymer- ligand with

collagen is modeled by using gencollagen package and the results of the modeling are given Figures 1 a & b. The figure ‘a’ represents ligand copolymer synthesized from collagen waste with various functional groups. It is seen from the figure that copolymer is composed of peptide bond (C=O-N-H) in the centre, projections of carboxylic (COOH) groups at the ends of the copolymer. These free functional groups play major role in adsorbing chromium as well as binding with collagen. Lone pairs of electrons of oxygen atoms in the molecule helps in interacting with chromium or collagen molecules. Collagen structure was modeled by using gencollagen package. The basic unit of the polymer was taken for the docking study which is simply chrome treated collagen molecule. The binding of copolymer with collagen is shown (Fig. 1a) along with various reactive sites present in the copolymer namely ‘RS1’ and ‘RS2’. The copolymer binds with the collagen with binding energy of -3.9 kcal/ mol (least binding energy was taken for further study). The copolymer binds with collagen through hydrogen bonding with the available O atom from free functional carboxylic acid groups of the copolymer and H


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atom of the collagen molecule with various bond lengths of 2.0, 2.1 and 2.3 Å at various binding sites as observed in the Figure 1b. The pending site represented as ‘US’ is unoccupied site ready for chrome adsorption from the aqueous medium. The ‘US’ site adsorbs chromium and provides additional stability to the collagen molecule. The docking of the copolymer

with collagen was

further studied for the complex

formation. The copolymer ready for docking and leading for formation of complex are given in Figure 2 a & b. ‘CBS’ and ‘CP’ represent the chromium bound sites and copolymer bound sites respectively in the collagen fibre. The copolymer adsorbs chromium and binds with the collagen at various reactive sites through hydrogen bonds and results in the fromation of more ‘CP’ complex sites. The ‘CP’ binding sites are clearly visible at many sites in the collagen leading to additional stability and improved adsorption of chromium in the collagen. The binding of ‘CP’ at various sites results in multipoint adsorption and fixation of chromium leading to Collagen-Cr-CP-Cr (C-Cr-CP-Cr) complex in the collagen moiety. This C-Cr-CP-Cr complex favors for the improved adsorption of chromium in the aqueous medium resulting in high performance adsorption of chromium to the level of 95.95 and 98.90% for the application of 6% copolymer in system. It is revealed from molecular modeling docking studies (Figure 1) that RS or US sites usually adsorb Cr and will bind with collagen. The adsorbent having Cr in the surface will tend to bind with co-polymer providing collagen-Cr-copolymer complex. Cr present in the surface of the adsorbent is chemically bound with collagen which will no more interfere with copolymer. These results are further confirmed by 1

HNMR and FT-IR studies.



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1

H NMR studies for the mechanism of adsorption

1

H NMR studies are very useful for knowing mechanism of adsorption and thereby rate

of adsorption of chromium with respect to functional groups into the complex. Water gate suppression method was applied to identify the functional groups of copolymer in the experimental samples. The results are shown in Figure 3. It can be observed from the figure that application of copolymer at the level of 3% in the tanning process showed peak at 8.384 ppm which is due to the presence of free functional group of carboxylic acid. Improved adsorption of chromium up to 93.58% has been achieved due to presence of those functional groups especially esters and free functional carboxylic group in the sample of 3% copolymer in the 1st cycle of tanning process. Adsorption improved to 96.85% in the 2nd cycle due to recycling. With increase in level of copolymer from 3 to 6%, the adsorption level increased to 95.5% in the 1st cycle and 98.90 in 2nd cycle of recycling indicating 6% as optimum value (As there were no significant increase in adsorption in the figure with 9% experimental sample). It can also be seen that height of peaks also reduced level with increase in % of copolymer in the sample indicating satisfaction of the reactive groups. Experimental sample with 9% copolymer showed relatively higher peaks due to excessive / unspent functional groups over that of 6% sample.

It is evident from the figure that with increase in percent of

copolymer, the samples show peaks of lesser heights due to saturation/ occupation of functional groups by the adsorbent chromium except in the case of experiment with 9% copolymer sample.


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FT-IR investigations The characteristic functional groups of the pure co-polymer were investigated through FT-IR analyses. The results are presented in Figure 4. The peak observed at 3390 cm−1 is due to presence of protein containing OH and NH groups. in the graft co-polymer. The peaks visible at 1715 cm−1 and at 1650 cm−1 are due to C=O stretching and to N-H bending frequency of the co-polymer. The functional groups present in the co-polymer envisaged that it is an amide co-polymer. It is further evident from the peak at 1340 cm−1 representing C-N stretching frequency of the amide group of the co-polymer. The C=O- and the N-H bonds present in the amide group provide stability to the collagen matrix by hydrogen bonding that is very helpful in the present investigation for the effective adsorbent of the chromium in the tanning process. Moreover, peak at 1555 cm1

represents presence of amide-II and 1644 cm-1 represents ester groups.

The experimental sample (application of 9% copolymer) showed a broad and strong characteristic band at 2860-3290 cm−1 for all the samples and is due to the stretching frequency of the protein containing O–H groups and NH groups for the graft copolymer. The band visible at 1600 cm−1 is linked to N-H bending of the sample (H-bonded) and the band visible at 1310 cm−1 is due to C-N stretching and the peak found 1115 cm−1 is due to C-O stretching and the peaks present at 815 cm−1 are due to out-of-plane bending of C-O-H for the experiment sample. The mechanism of the adsorption of chromium by the copolymer was further substantiated by mathematical modeling. Shifted peaks seen at 1036 cm-1 for experimental copolymer sample (Figure 4(a)) are due to presence of Cr=O while shifted peaks at 1381 cm-1 for experimental sample



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(Figure 4a) is due to presence of Cr-N= . Thus it is evident that both carboxylic acids and amide groups are involved in the adsorption of Cr. Mathematical Modeling Chromium containing waste has been hydrolyzed (CH) by alkali and copolymerized with PEG to synthesize graft copolymer, an efficient auxiliary for chrome tanning. It is assumed that copolymer, the complex molecules slowly gets transported to collagen matrix where carboxylic acid group sites are available. The complex tanning agent is in aqueous medium while the collagen matrix has a solid phase with an interface between solid and aqueous boundary. It is assumed that during the transport, concentration of complex tanning agent is constant and uniform (due to constant stirring) in the aqueous phase followed by a depletion in the interface which further depletes due to slow diffusion across solid phase of collagen matrix. Some of the molecules get adsorbed on the surface of collagen forming a monolayer (as confirmed by Langmuir isotherm) which then slowly diffuse through pores of collagen towards the active sites and latter gets bounded with –COOH groups to form stable complex which completes the reaction. In a reaction between active sites of complex (C) and collagen (B) to form /stabilize a product P, the reaction becomes C + B → P

(1)

Rate of reaction r = −kCC CB

(2)

According to Langmuir sorption isotherm, CC =

[C ]CC 0 1 + [ B ] CC 0

(3)

This equation gives adsorption capacity. If reaction becomes almost complete, concentration of unbound reactants become very low, making [B]CC0 term of denominator very less or small so that Eqn (3) reduces to


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CC = [C ] CC 0

(4)

This diffusion followed by reaction can be described by

D

∂ 2CC ( x,τ ) ∂CC ( x,τ ) = + r ( x,τ , CC ) ∂x 2 ∂x

0≤x≤L

With boundary conditions as CC ( L,τ ) = ε C0 (τ )

− DS

∂CC ( L,τ ) ∂x

CC ( x, 0 ) = 0

= V0

and

(5)

∂CC ( 0,τ ) ∂x

=0

∂CC 0 (τ )

(6)

(7)

∂τ

, CC 0 ( 0 ) = C0

(8)

Equation (5) describes diffusion of components from bath towards collagen in the direction of washing liquid. Boundary conditions are due to perfectly mixed state and symmetric field or gradient of concentration inside the collagen matrix. The diffusion flux at boundary causes transport of the complex towards collagen is represented by Equation (7). Respective boundary and initial conditions are given in Equation (8). Initial distribution of reactive complex in the solid is represented by boundary condition while initial condition provides information on initial concentration of the reactive complex. Substituting term for rate of reaction and introducing dimensionless variables in Equation (5) one can get

K

∂ 2CC ∂CC = ∂X 2 ∂τ

where K =

D 1 + k2

0≤ x≤L

,

,

C=

C − C0 C P − C0

,

,

τ >0

X=

x L

,

(9)

F0 =

V Kτ and NV = 0 2 L V



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Page 20 of 46

and k2 = coefficient of adsorption, D is diffusion coefficient, thickness or depth of the skin or collagen be 2L and Nv is the dimensionless number. Analytical solution of Equation (9) becomes

C ( X , F0 ) =

ε (1 + k2 )

ε (1 + k2 ) + NV

cos ( an X ) exp ( −an2 F0 )

∞

− 2 NV ∑ n =1

ε (1 + k2 ) cos ( an ) −

ε (1 + k2 ) an

(10)

sin ( an ) − NV an sin ( an )

which can be written as   2  ε 1+ k  ∞ cos ( an X ) exp ( − an F0 ) ( 2 ) − 2N  C ( X ,τ ) = C0 P + ( CP − CoP )  V∑ ε (1 + k2 )  ε (1 + k2 ) + NV  n =1 sin ( an ) − NV an sin ( an )  ε (1 + k2 ) cos ( an ) −  an   (11)

where an are the roots of equation tan ( a ) =

At steady state, F0 = 1 or τ s =

NV a ε (1 + k2 )

(12)

L2 K

Equation (11) can be solved to compute concentration profile of the complex along the collagen matrix. After solving equation (11), concentration of chromium can be plotted with respect to time and depth of penetration/ diffusion. Initial concentration of chromium in bath is known and from the depletion amount, it is possible to know amount penetrated in each time inside the skin. Figure 5 shows the profile for 3% sample for chromium inside skin. The skins having 5 mm thickness were added to 2 l of float and chromium adsorption


Page 21 of 46

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coefficient (k2) was found to be 0.1098, diffusion constant of 3.0E-09 has been considered for calculation. From Eqn(3), the adsorption capacity (CC0e=qe) can be found as 0.7063 g/g of adsorbate, separation factor

1 = 0.0082 and Biot number 1 + k2 ce

becomes 12.04. while fitting a predicted model (Figure 6a) with experimentally founded adsorbed amount of particles, a R2 value of 0.94 was found to hold good. Figure 5a shows Langmuir isotherm for absorption of Cr with 9% co-polymer. Linear nature of the curve confirms Langmuir isotherm. Optimization of operating parameters (%Copolymer, pH, Duration)

studies using response surface method reveal optimum

conditions as 6.6% co-polymer, 3.9 pH and 6.0 hr duration for optimum adsorption of 96% Cr. Figures 6b and 6c explain the absorption under the influence of various operating parameters.

It can be seen that exhaustion increases with time while

concentration does not vary with depth of skin. Authors used cellulose-clay composite that exhibited adsorption capacity of 0.022 g g-1 of adsorbent. They fitted the data using Langmuir sorption isotherm and claimed that the composite material could be regenerated using sodium hydroxide as eluent in 10 adsorption-desorption cycles.

46

Raw fleshings were complexed with Iron to remove Cr(VI) from effluent. The adsorption capacity of the material was found to be 0.051 g g-1 of adsorbent which increased the capacity by 10 folds.

31

Other researcher reported Cr(III) adsorption as precipitation with

struvite as 22. 2-3030.1 mg/kg.

40

The rate of adsorption of chromium by the copolymer

and the participation of chromium and copolymer is further studied by SEM, SEM- EDX and AFM techniques.



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Page 22 of 46

SEM and SEM-EDX Analyses Morphology of the chromium adsorbed co-polymer has been studied under SEM micrographs. The samples have been prepared at different concentrations and are subjected to different magnifications. Experimental sample obtained by treating copolymer (at various concentrations such as 3, 6 and 9%) and pure copolymer are put under magnification 100 µmX315, 20µmX1.150,

20µmX1.360K, 20µmX1.59K

respectively to obtain clear morphology with better resolution. In all the SEM Figures (Figure S2) it can be found that there are two types of grains present. One has thick conjugated fibrous structure representing the co-polymer and another smaller globular crystalline structure showing presence of chromium. SEM picture of the pure polymer sample shows long fibrous structure having free carboxylic acids as functional groups for adsorbing chromium. The experimental sample obtained by treating copolymer at 3% shows that chromium has been adsorbed on the fibrous structure of the copolymer. It is evident from the figure the active sites of co-polymers are occupied by chromium as seen by globular grain structure embedded in the fibrous structure of the co-polymer in the image leaving some unoccupied active sites of the co-polymer. Similarly the experimental sample obtained by 6% co-polymer shows that chromium has almost been occupied by reactive sites of the co-polymer which is confirmed in the figure.

Further to be noticed that,

smaller granules of chromium particles are adsorbed by the reactive functional groups in the micrographs. The experimental sample that has been treated with 9% co-polymer shows that complete active sites of co-polymer are satisfied by chromium (as seen by globular grain structure embedded in the fibrous structure of the co-polymer in the


Page 23 of 46

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image) and no empty / unoccupied fibrous structure are visible, indicating maximum adsorption of chromium. Chromium adsorbed by co-polymers has been found-out quantitatively through SEMEDX and results are presented along with chromium (Cr) peak in Figure 7 a-d. The overall results obtained from SEM-EDX confirmed that increase in treatment of copolymer in the experiment increases the adsorption of chromium. SEM-EDX of the polymer sample shows stratified layers of fibrous structure with two elements, namely, C and O with

weight

% of Cr at 56.55, 43.45 while atomic % is 63.42, 36.58

respectively. It is seen from the figure that treatment of copolymer at 3% shows weight % of chromium at 17.36 while atomic % is 7.94 indicating relatively higher amount of chromium is present in the present in the sample confirming relatively lower amount of chromium had been adsorbed in the co-ordinated complex. The treatment of copolymer at 6% level shows weight % of chromium at 6.33 while atomic % is 2.03 indicating relatively lower amount of chromium present in the sample confirming relatively more amount of chromium had been adsorbed in substrate. SEM-EDX curves for the treatment of copolymer at 9% shows no traces of chromium confirming complete adsorption of chromium has been taken place leaving no chromium in the effluent. This is one of the rare investigations that show the copolymer as a high performance adsorbent which can be able to adsorb more amount of chromium from the bath through functional groups of carboxylic acid sites. Thus it is justified that copolymer has acted as high performance adsorbent for chromium.



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Page 24 of 46

AFM analysis AFM analysis has been carried-out for the samples and for pure co-polymer sample using different magnifications (2, 3, 8, 25 µm and 800 nm) to know the topography and deflections of samples. The result of the adsorption of chromium by copolymer is given in Figures 8 (a-d). The AFM result of pure copolymer sample is presented in Figure 8d. It can be seen from this figure that one type of particles are present as randomly distributed crystalline projections which are due to copolymers.

The deflection image

shows copolymer particles nonuniformly distributed that is ready for adsorption of chromium. This copolymer sample contains free functional groups of carboxylic side chains that are active to adsorb chromium. This arrangement of structuire of copolymer with more number of atoms provides lack in stiffness. Similarly the line graph representing deflection of mean fit shows almost uniform distribution except at the latter stage (near 25 µm) where some spikes are visible with less spread of higher size of particles. Experiment carried-out with the help of copolymer at the level of 3% in chrome tanning process is given in Figure 8a. The AFM figure shows two types of particles present on the image where one is due to chromium particles representing small-darker globular structure and others are due to copolymers. Availability of less number of copolymer grooves hint that most of the copolymers have been taken up by the collagen during tanning. It is further evident from the deflection image that the copolymer has adsorbed the chromium from aqueous environment which finally bound with the collagen matrix leading to collagen-copolymer-Cr complex. The corresponding line graph showed in Figure 9a for deflection of experimental sample show uniform distribution of mean fit.


Page 25 of 46

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Few spikes are due to narrow distribution of copolymer particles left unadsorbed during tanning. The line graph representing the surface fit of deflection image shows uniform distribution of copolymer adsorbed with chromium about the mean fit. Next levels of experiments have been carried out to with 6% copolymer to know the rate of adsorption of chromium using AFM study. The topography and deflection of the sample is shown in Figure 8b. The topography shows some marks of bigger whitish chunks which are due to copolymers. It is evident from the topography that chromium is almost not visible indicating increased rate of adsorption compared to earlier sample (3%). The subsequent line graph of mean fit of experimental sample carried out with the help of copolymer at 6% level represents deflection pattern showing evenly distribution around mean value indicating a regular smooth surface confirming better adsorption of chromium (Figure 9b). In continuation with the investigation for studying improved adsorption of Cr, a sample with 9% copolymer is subjected to AFM image which is presented in Figure 8c. It is evident from the deflection image that there was absence of chromium spots leaving the presence of copolymer particles alone indicating complete adsorption of chromium by the functional groups of the copolymer; therefore the experiment witnessed maximum amount of adsorption of chromium.

The deflection image also shows almost no

chromium left in the bath. The increase in adsorption of chromium had resulted in surface roughness of the sample which is prominent in topography image. The line graph of mean fit of experimental sample carried out with the help of application of copolymer at 9% level represents deflection pattern showing evenly distribution around mean value indicating a better adsorption of chromium as compared to earlier



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Page 26 of 46

experiments (with 3 and 6% of copolymer) with increased regular smoothness of surface of the sample (Figure 9c). It is inferred from the AFM result that increase in amount of copolymer increased the level of adsorption of chromium which agrees with the 95.95% of chromium adsorption in the experimental sample. CONCLUSION A graft copolymer has been synthesized from collagenous waste and PEG for studying improved adsorption of chromium in the chrome tanning process. The synthesized copolymer has been characterized as having particle size of 310 nm, pH of 4.0, relative viscosity of 0.8872 cp, polydispersity index (Mw/Mn) as 0.555 and % solid level as 5.05 using standard methods. The zeta potential of copolymer showed –0.0403 mV indicating weakly anionic nature suggesting a slight counter-productive over the isoelectric point of collagen favoring enhanced adsorption of chromium. Two batches of continuous chrome tanning process with recycling has been carried out to study adsorption behavior of the copolymer whereby it was found that maximum % adsorption of chromium of 95.95 using 6% level of co-polymer. Molecular modeling using gencollagen package revealed mechanism of improved adsorption of chromium by the copolymer and binding of copolymer ligand with collagen with binding energy of - 3.9 kcal/mol through hydrogen-bonding with bond lengths of 2.0, 2.1 and 2.3 Å for the various active sites. 1H NMR studies showed reduction in peak at 8.384 ppm (6% copolymer) for the maximum occupancy of free functional carboxylic acid groups that play main role in adsorbing chromium. Esters resulted from grafting masks chromium complexes thereby increasing stability of adsorption.

Diffusion controlled material

balance represented by partial differential equation is solved to find concentration profile


Page 27 of 46

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as functions of time and depth of penetration. SEM-EDX studies confirmed increased adsorption of chromium by copolymer by showing 6.33 weight

% of chromium

containing atomic % as 2.03 indicating relatively lower amount of chromium present in the sample confirming relatively more amount of adsorbed chromium by substrate. Topography and deflection from AFM study clearly illustrate complete adsorption of chromium using 9% copolymer as evidenced from absence of chromium spots on the surface. Surface smoothness, softness and improved adsorption have been confirmed by uniform spread of mean fit curve of AFM.

Thus application of copolymer with

continuous recycling provides new insight of high performance adsorption in managing Cr(III). ASSOCIATED CONTENT Supporting Information Methods describing estimation of chromium, 1H NMR spectra with water suppression using watergate sequence, FT-IR analysis, mathematical modeling, SEM, SEM- EDX studies, AFM study have been provided in the supporting information. Table S1 represents characteristics of grafted copolymer, while, Table S2 represents percentage of grafting and grafting efficiency of graft PEG on CH. Figure S1 representing characterization of graft copolymer a) TGA; b) DSC; c) zeta potential; d) Particle size and Figure S2 representing SEM analysis have also been provided in the supporting Information. This

material

is

available

free

of

charge

http://pubs.acs.org. AUTHOR INFORMATION Email: [email protected]


via

the

Internet

at


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Page 28 of 46

Notes The author declares no competing financial interest. Acknowledgement The authors would like to acknowledge the support of Dr. V. Subramaniyan, senior principal scientist and his team for the valuable support in carrying out computational study. REFERENCES (1) Liu, C.C.; Wang, M.K.; Chou, C.S.; Li, Y.S.; Lin, Y.A.; Huang, S.S. Chromium removal and sorption mechanism from aqueous solutions by wine processing waste sludge. Ind. Eng. Chem. Res. 2006, 45, 8891-8899. (2) Nam, K.H.; Tavlarides, L.L. Synthesis of a high-density phosphonic acid functional mesoporous adsorbent: Application to chromium (III) removal. Chem. Mater. 2005, 17, 1597-1604. (3) Yoshitake, H.; Yokoi, T.; Tatsumi, T. Adsorption of chromate and arsenate by amino-functionalized MCM-41 and SBA-1. Chem. Mater. 2005, 14, 4603-4610. (4) Sangyun, Y.; Park, D.; Park, J. M.; Milvolesky, B. Biosorption of trivalent chromium on the brown seaweed biomass. Environ. Sci. Technol. 2001, 35, 4353. (5) Kanagaraj, J.; Senthilvelan, T.; Panda, R. C; Aravindhan, R.; Mandal, A. B. Biosorption of trivalent chromium by using Bacillus pumulus isolated from chromium contaminated soil. J. Chem. Engg. Tech. 2014, 37, 1741.


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(6) De Castro Dantas, T. N.; Dantas Neto, A. A.; De A. Moura, M. C. P.; Barros Neto, E. L.; Telemaco, E. D. P. Chromium adsorption by chitosan Impregnated with microemulsion. Langmuir 2001, 17, 4256. (7) Walkley, A.; Black, C. A. An experimentation of Detjareff method and a proposed modification of the chromic acid titration method. Soil Sci. 1934, 37, 29. (8) Rao, J.R.; Thanikaivelan, P.; Sreeram, K.J.; Nair, B.U. Green route for the utilization of chrome shavings in tanning industry. Environ. Sci. Technol. 2002, 36, 1372-1376. (9) Taylor, M.M.; Diefendorf, E.J.; Na, G.C. Enzymic treatment of chrome shavings. J. Amer. Leather Chem. Assoc. 1990, 85, 261–282. (10) Kanagaraj, J.; Senthilvelan, T.; Panda, R.C.; Kavitha, S. Eco-friendly waste management strategies for greener environment towards sustainable development in leather industry: a review. J .Clea. Prodn, 2015, 89, 1-17. (11) Kanagaraj, J.; Chandra Babu, N. K.; Mandal, A. B. Recovery and reuse of chromium from chrome tanning waste water aiming towards zero discharge of pollution. J. Clea. Prodn. 2008, 16, 1807. (12) Cot, J.; Marsal, A.; Manich, A.; Celma, P.; Choque, R.; Cabeza, L.; Labastida, L.; Lopez, J.; Salmeron, J. Minimization of industrial wastes: adding value to collagen materials. J. Soc. Leather Technol. Chem. 2003, 87, 97. (13) Kolomaznik, K.; Mladek,M.; Langmaier,F.; Janacova, D.; Taylor. M. M. Experience in industrial practice of enzymatic dechromation of chrome shavings. J. Amer. Leather Chem. Assoc. 1999, 94, 55-63. (14) Back, J.Y.; Yu, H.; Song, I.; Kang, I.; Ahn, H.; Shin, T.J.; Kwon, S.K. Investigation



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of

structure−property

semiconductors

via

relationships side-chain

in

Page 30 of 46

diketopyrrolopyrrole-based

engineering.

Chem.

Mater.

polymer

2015,

DOI:

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Thermolytic transformation of tris (alkoxy)

siloxychromium (IV) single-source molecular precursors to catalytic chromia-silica materials. Chem. Mater. 2001, 13, 1817-1827. (17) Qiaxian, C. S.; Xinxin, L.; Wenhua, Z.; Xuepin, L.; Bi, S. Raw skin wastes–used to prepare a collagen fibre adsorbent for the chromatographic separation of flavanoids. J. Soc. Leather Technol. Chem. 2014, 98, 93-98. (18) Kresalkova, M.; Hnankova, L.; Kupec, J.; Kolomaznik, K. Application of protein hydrolysate from chrome shavings for polyvinyl alcohol based biodegradable material. J. Amer. Leather Chem. Assoc. 2002, 7, 143-149. (19) Hervas, F.F.; Celma, P.; Punti, Cot,J.; Marshal, A.; Manich. Enzyme ability of trypsin on skin in a two step collagen extraction process. J. Amer. Leather Chem. Assoc. 2007, 102, 1-9. (20) Catalina, M.; Antunes, A.P.M.; Attenburrow, G.; Cot, J.; Covington, A.D. Sustainable management of waste-reduction of the chromium content of tannery solid waste as a step in the cleaner production of gelatin. J. Sol. Waste Technol. Manage. 2007, 33, 43-50.


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Sadulla, S.; Prasada Rao, B. High exhaust tanning systems

using a novel cross-linking agent. J. Soc. Leather Technol. Chem. 2005, 90, 127. (24) Rao, J.R.; Thanikaivelan, P.; Sreeram, K.J.; Nair, B.U. Green route for the utilization of chrome shavings in tanning industry. Environ. Sci. Technol. 2002, 36, 1372-1376. (25) Trott, O.; Olson, A.J. Auto Dock Vina: improving the speed and accuracy of docking with a new scoring function, efficient optimization and multithreading. J. Comput. Chem. 2010, 31, 455–461. (26) Huang, C. C.; Couch, G. S.; Pettersen, E. F.; Ferrin, T. E.; Howard, A. E.; Klein, T. E. The object technology framework: an object-oriented interface to molecular data and its application to collagen. Pac. Symp. Biocomput. 1998, 349–361. (27) DeLano, W.L. The Pymol molecular graphics system, DeLano scientific LLC, Palo Alto, CA, 2008. (28) IUC 8, Determination of chromic oxide content. J. Soc. Leather Technol. Chem. 1998, 82, 200. (29) Echlin, P. In Scanning Electron Microscopy, Vol. 4; Heywood, V. H., Ed.; Academic Press: London, 1971, 307.



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Takovite−Aluminosilicate−Cr materials prepared by adsorption of Cr 3+ from industrial effluents as catalysts for hydrocarbon oxidation reactions. ACS Appl. Mater. Interfaces 2012, 4, 2525. (38) Jianzhong, M.; Lingyun, L.; Chunhua, X.; Wenqi, W.; Zongsui, Y. Protein retanning and filling agent from vinyl monomer graft modification of chrome shavings Hydrolysate. J. Soc. Leather Technol. Chem. 2003, 88, 1. (39) Aslan, G. I.; Gulumser, G.; Oclak, B.; Aslan, A. Use of silk hydrolysate in chrome tanning. J. Soc. Leather Technol. Chem. 2013, 98, 193. (40) Rouff, A. A. Sorption of chromium with struvite during phosphorus recovery. Environ. Sci. Technol. 2012, 46, 12493. (41) Kanagaraj, J.; Panda, R. C.; Sumathi, V. Synthesis of graft-copolymer adsorbent through green route and studies on its interactions with chromium (III) through active functional groups: kinetics and improved adsorption supported by SEM-EDX and AFM. RSC Advances 2015, DOI: 10.1039/C5RA05799J (42) Mola, E.E.;Coronel, E.; Joly, Y.; Vicente, J.L. Bonding studies of chromiumnitrogen molecules. Langmuir 1988, 4, 917–920. (43) Pamfil, D.; Schick, C.; Vasile, C. New hydrogels based on substituted anhydride modified collagen and 2-hydroxyethyl methacrylate synthesis and characterization. Ind. Eng. Chem. Res. 2014, 53, 11239. (44) Degenhardt, J.; McQuillan, A.J. In situ ATR-FTIR spectroscopic study of adsorption of perchlorate, sulfate, and thiosulfate ions onto chromium (III) oxide hydroxide thin films. Langmuir 1999, 15, 4595–4602.



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(45) Bhaskar, G.; Mandal, A. B.;

Page 34 of 46

Ramasami T. Synthesis, characterization and

micelle formation in an aqueous solution of methoxy ploy (ethylene glycol) macromonomer, homopolymer and graft copolymer. Macromolecules 1993, 26, 4083. (46) Kumar, A. S. K.; Kalidhasan, S.; Rajesh, V.; Rajesh, N. Application of celluloseclay composite biosorbent toward the effective adsorption and removal of chromium from industrial wastewater. Ind. Eng. Chem. Res. 2012, 51, 58.


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Table 1. % Adsorption of chromium in chrome tanning process using copolymer. % of copolymer

Cycles

% Cr2O3 (per litre)

offered

% Adsorption of Cr

3%

First cycle

2.597 gm

93.58

6%

First cycle

1.1822 gm

95.95

9%

First cycle

1.671 gm

95.88

3%

Chromium recovered

0.163 gm

-

0.3724 gm

-

0.162 gm

-

from the first cycle 6%

Chromium recovered from the first cycle

9%

Chromium recovered from the first cycle

3%

Second cycle

1.270 gm

96.85

6%

Second cycle

0.490 gm

98.90

9%

Second cycle

0.420 gm

98.82



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Scheme 1. Schematic flow chart for the 1st and 2nd cycle of chrome tanning with copolymer


Page 36 of 46

Page 37 of 46

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RS1 RS2

(a)

US

1

-3.9

0.000

0.000

2

-3.7

2.442

3.508

3

-3.7

9.527

11.505

4

-3.6

11.157

12.563

5

-3.6

11.449

12.948

6

-3.6

12.520

13.672

7

-3.6

26.030

27.787

8

-3.6

27.508

29.339

9

-3.6

19.687

21.248

(b)

Figure 1. Molecular modelling of docking of copolymer with chromium treated collagen a) ligand copolymer. RS1 and RS2 indicate the reactive sites of carboxylic acid. These sites play important role in adsorbing chromium and binding with collagen. b) docking of copolymer ligand with collagen with binding energy of -3.9 kcal/mol as observed in the Figure. US represent unoccupied sites that are ready for adsorption of chromium in the environment.



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Page 38 of 46

CBS CP RS

(a)

CBS CP

CP

CP CP CBS (b) Figure 2. Molecular modelling of docking of copolymer with the collagen matrix. a) Represents collagen with reactive site (RS) ready for docking and CBS represents the chromium bound site whereas CP is copolymer site ready for docking with collagen. b) Represents docked copolymer with collagen indicating more copolymer as binding site (CP) with already existing chromium bound site (CBS). This figure illustrates that more docking of CP sites lead to increased adsorption of chromium.


Page 39 of 46

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RCOOH

a)

RCOOH

b)

RCOOH c)

d) Figure 3. 1HNMR analysis for the various experimental samples (a-c) and control sample/ without treatment of copolymer sample (d). Water Gate suppression method was used to investigate the presence of functional carboxylic acid sites. All the experimental samples show the presence of carboxylic acid groups at 8 ppm. Water peak has been suppressed at 4 ppm to elucidate the functional group.



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(a)

(b)

Figure 4. FT-IR spectra of the (a) 9% copolymer treated sample in the adsorption of chromium; (b) pure copolymer sample.


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Figure 5. Concentration profile of penetration of chromium through skin.



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(a)

(b)

(c) Figure 6. (a) Adsorption kinetics of Cr confirming Langmuir isotherm (b) Adsorption profiles due to variation of pH and different % of Co-polymer (c) Adsorption profiles due to variation of duration of exhaustion and different % of co-polymer.


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Element

Weight%

Atomic%

OK

34.26

50.94

Na K

23.84

24.66

Cl K

24.54

16.46

Cr K

17.36

7.94

Totals

100.00

Element

Weight%

Atomic%

CK

30.10

41.71

OK

40.05

41.66

Na K

13.97

10.11

Cl K Cr K Totals

9.56 6.33 100.00

4.49 2.03

Element

Weight%

Atomic%

OK

32.47

46.97

Na K

25.34

25.50

Cl K

42.19

27.53

Totals

100.00

Element

Weight %

Atomic %

CK

56.55

63.42

OK

43.45

36.58

Totals

100.00

(a)

(b)

CP

(c)

CP

(d) Figure 7. SEM-EDX analysis (t-b) a) 3% copolymer treated sample; b) 6% copolymer treated sample; c) 9% copolymer treated sample; d) pure copolymer sample. Images of experiment carried out with treatment of 3% copolymer sample and 6% copolymer sample show presence of chromium where as image of 9% copolymer-treated-sample show absence of chromium indicating almost complete adsorption of chromium.



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Cr

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CP

(a)

Cr

CP

(b)

CP

(c)

CP (d) Figure 8. (t-b) Topography and deflection of AFM photographs. (a) 3% co-polymer treated sample (b) 6% co-polymer treated sample (c) 9% co-polymer treated sample (d) pure co-polymer sample in the chrome absorption process.


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.

(a)

(b)

(c)

(d)

Figure 9. (t-b) Deflection and mean fit for copolymer treated sample with 3, 6 and 9% and for pure copolymer sample (a-d). The 3d deflection of the 3, 6% copolymer treated sample showed presence of chromium and 9% showed bigger spikes indicating presence of copolymer.



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TOC Art


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Water-Soluble Graft Copolymer Synthesized from ... - ACS Publications

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