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In this review, we selected the published data related to the chemical modification like grafting, cross-linking, esterification, etherification and dual modification of starch and appli

Chemical modification of starch and its application as an adsorbent

material

Muhammad Haroon, Li Wang∗, Haojie Yu*, Nasir M Abbasi, Zain-ul-Abdin, Muhammad Saleem, Rizwan Ullah Khan, Raja Summe Ullah, Qing Chen, Jialiang

Wu

State Key Laboratory of Chemical Engineering, College of Chemical and Biological

Engineering, Zhejiang University, Hangzhou 310027, P.R China

Abstract

Starch is a biopolymer of the plant origin which is cheap abundant and has many

applications in food and non-food industries However, in the native form, its

applications are limited due to shortcomings, such as loss of viscosity and thickening

power upon cooking and storage, retrogradation characteristics and absence of certain

groups responsible for a particular function, etc So, in order to reduce its limitations

and improve its applications, modification of starch is necessary It can be modified

by several ways like chemical modification, physical modification and genetic

modification but the most important one is the chemical modification In this review,

we selected the published data related to the chemical modification like grafting,

cross-linking, esterification, etherification and dual modification of starch and

application of modified starch for the adsorption of organic dyes and heavy metals

A naturally occurring biopolymer, starch is a cheap, biodegradable, renewable and

abundantly available polysaccharide molecule, which is obtained from plants.1 Starch granules are made of mainly two kinds of alpha-glucan, amylose and amylopectin,

which are about 98-99 % of the total net weight of the starch Amylose is a

comparatively linear α-glucan which has 1 % α(1-6) and 99 % α(1-4) linkages while

amylopectin has extremely branched structure having about 5 % 6) and 95 %

α(1-4) linkages Small amount of proteins and lipids are also present in starch 2, 3Naturally occurring starch has limited industrial applications due to insolubility in

water at room temperature, easy retrogradation and instability of its pastes and gels

The functionality of starch can be modified by a several ways like chemical

modification, genetic modification and physical modification.1 In this review, we have focused on the chemical modification of starch The most important use of the

chemically modified starch is its use as an adsorbent for the removal of dyes and

heavy metals The major contaminating sources of heavy metals are metallurgy,

electroplating industries, industrial sewage and household sewage.4 These metals cause renal tubular damage, cancer, hyperkeratosis, anxiety and depression, irritation

and damage to the nervous system in human beings5 and cardiovascular, hematologic, reproductive, metabolic and endocrine disturbances, necrosis, restricted growth, skin

lesions, and hypocalcaemia in fish.6 Similarly in our modern industrial society, many industries use dyes to color their

products.7 These dyes in an effluent, even in a small amount can have harmful effects, not only on the environment, but also on living organisms In addition, some dyes and

their degradation products are carcinogenic and toxic These dyes are important

sources of water pollution and their treatment becomes a major problem for

Usually, heavy metals and dyes are removed from wastewater by flotation, chemical

precipitation, electrochemical deposition, ion exchange and adsorption However, the

processes other than adsorption have certain limitations like waste of chemicals,

sludge production, poor settlement and non-selectiveness So, adsorption is one of the

best techniques used for the removal of heavy metals and other wastes from

wastewater The process for removing waste on sorbents requires three main steps In

first step, the adsorbate particles migrate from solution to the surface of sorbent In

second step these particles get adsorbed on the surface and in third step further

movement of these particles within the sorbent particles occurs.9 Activated carbon is considered as a good adsorbent because of its large surface area and outstanding

adsorption property, but its use is limited due to its high cost, non-selective adsorption

and regeneration problems.10-12 Mostly, the synthetic polymers used for the removal

of heavy metals are non-biodegradable and non-renewable and may act as secondary

pollutants So, these synthetic polymers are not environmental friendly adsorbents

Starch, a plant biopolymer is considered to be the excellent substitute comparing with

activated carbon and other synthetic polymer adsorbents because it is biodegradable

and environmentally safe However, native starch can’t be used directly as an

adsorbent due to its no adsorption ability for heavy metals and most of the dyes In

order to make starch as good adsorbent for heavy metals and dyes, there is a need to

modify native starch by the introduction of active groups like xanthate, carboxylate,

acrylate, amine phosphate and many other groups, which have chelating ability.13Dithiocarbamate starch (DTCS),14 porous starch citrate (PSC), porous starch xanthate (PSX)15 and etherified corn starch containing maleic acid and itaconic acid16 have been used for the adsorption of heavy metals from water These modified starches are

removal of these metals A novel amphoteric starch having quaternary ammonium and

phosphate groups has been effectively utilized for cationic and anionic contaminants

treatment.17 Similarly, magnetic nanocomposite hydrogel (m-CVP) beads, prepared

by cross-linking the mixture of carboxymethyl starch-graft-polyvinyl imidazole

(CMS-g-PVI), poly(vinyl alcohol) (PVA) and Fe3O4 with glutaraldehyde (GA) in boric acid, have been utilized for the removal of congo red (CR) and crystal violet

(CV) dyes and some transition metal ions like Cu+2, Pb+2 and Cd+2.18 Cross-linked amphoteric starch having quaternary ammonium and carboxymethyl groups has been

used for the removal of acid and basic dyes Acid dyes were removed by ammonium

group, while basic dyes were removed by carboxymethyl group.19The purpose of the modification of starch is to enhance the useful properties (like

adsorption) of starch and to reduce its unwanted properties.20 Although, some review papers have been published which describe the modification and applications of

starch,1, 21 but some aspects of chemical modification and applications of the modified starch are still not described in detail In this review, we have focused our

discussion on the chemical modification of starch and its application as an adsorbent

material for the removal of different chemical dyes and heavy metals from

wastewater

2 Chemical modification

The introduction of new functionality in the starch is called chemical modification of

starch The new functionality may be carboxyl, acetyl, hydroxypropyl, amine, amide

or any other functional group which gives specific properties to the starch The

presence of a large number of hydroxyl groups on starch provides more reactive sites

for the chemical modification of starch Studies related to the chemical modification

of starch have been started in early 1940s There are various methods of chemical

modification of starch, but some important methods are grafting, cross-linking,

etherification, esterification and dual modification.21

2.1 Grafting

In this process, monomers are covalently bonded to the main polymer chain and then

further polymerized on this chain The time of grafting is variable and it may takes

minutes, hours and sometime days to complete.22 Like other biopolymer, starch is also graftified for various applications in different fields like drug delivery, tissue

engineering and wastewater treatment Generally three approaches grafting onto,

grafting from and grafting through are used for synthesis of graft co-polymers

Grafting onto approach is related to the reaction between functional groups of two

different polymers Grafting from approach is referred to the grafting in which a

polymer with specific functional group triggers the polymerization of vinyl

monomers Grafting through approach involves copolymerization of

macromonomers.23 Among these approaches, grafting from approach is the most frequently used technique, because of its high grafting yield, which is due to easy

access of the reactive groups to the chain ends of the growing polymers.24 The

different types of grafting are shown in the flowing sheet diagram (Fig 1).22, 25

Basically grafting follows three reaction paths, free-radical path, ionic path and living

Fig 1 Flow sheet diagram of grafting.21, 24

2.1.1 Free-radical grafting

Free-radical grafting (FRG) is the most important and most commonly used method

of grafting.21 It is the easiest and economical method for modification of biopolymers for different applications like wastewater treatment, tissue engineering, drug delivery

and food additives On the basis of initiators required to start FRG, it is further

divided into following three types

2.1.1.1 Grafting induced by chemical initiators

In this type of grafting, usually, vinyl monomers are grafted onto biopolymers

initiated by chemical initiators The different chemical initiators used are ceric

ammonium nitrate (CAN), cerium sulphate (Ce2(SO4)), ceric ammonium sulfate (CAS), Fenton’s reagent (Fe+2 + H2O2), Co (II) potassium monopersulfate, Co (III) acetylacetonate complex salts, azobisisobutyronitrile (AIBN), potassium persulfate

(KPS) ammonium persulfate (APS) and benzoyl peroxide (BPO).22, 26 Among redox initiators, CAN is the most commonly used initiator because it results in the product

with high grafting efficiency and low amount of homopolymer formation The general

synthetic rout of grafting of vinyl monomer on starch is given in Scheme 1 Nair et al

prepared cassava starch-graft-polymethacrylamide (St-g-PMAM) using CAN as a free

radical initiator The maximum grafting percentage (79.9 %) was obtained, when

0.878 g/L CAN was used for grafting 20 g of methacrylamide (MAM) on 10 g of

starch and the reaction was carried out for 2 h at 55 ˚C.27 Lele grafted potato starch with acrylic acid (AA) using CAN as an initiator.28 Apopei et al found that potato starch grafted with acrylonitrile using two initiators system ((Ce(SO4)2 and CAN) showed three times higher grafting percentage than using single initiator (CAN).29

Mishra et al prepared starch-graft-polyacrylamide (St-g-PAM) by using microwave

radiations combined with CAN as radical initiators This method resulted in

qualitative product with better grafting yield than the methods in which only chemical

initiators were used.30

Scheme 1 General synthetic route for the grafting of vinyl monomer on starch

induced by Ce+4.31

This grafted polymer acted as superabsorbent for the removal of heavy metals.32 The comparative mechanism of grafting of acrylamide on starch by CAN with and without

microwave assistance is given in Scheme 2 Witono et al carried out grafting of

cassava starch with AA using Fe+2/H2O2 redox system as a radical initiator Grafting efficiency was found to depend on concentration of starch, temperature and starch to

monomer ratio.33 Grafting of acrylic acid on starch using Fenton’s reagent is given in

Scheme 3

Scheme 2 Grafting of acrylamide on starch by CAN: (a) with microwave irradiation

and (b) without microwave irradiation.34

Scheme 3 Grafting of starch with acrylic acid using Fenton’s reagent: (a) grafted

product and (b) homopolymer (a side product).33

Mohammed et al synthesized a superabsorbent grafted polymer of potato starch by

grafting acryloylated starch with AA in the presence of same radical initiating system

(Fe+2+H2O2) The product synthesized by this method had lower homopolymer concentration and higher adhesive and film forming properties than the copolymer

formed by direct grafting of AA on starch.35 Synthesis of acryloylated

starch-graft-poly (acrylic acid) is shown in Scheme 4 Guo et al used KMnO4, HIO4, and H2SO4for grafting AM on starch With this system, grafting yield and grafting efficiency

were increased and the homopolymer content was decreased in comparison with

KPS and BPO, KSP resulted into the higher grafting yield than the other two.37

Grafting of acrylic acid on hydrolyzed starch is given Scheme 5

Scheme 4 Synthesis of acryloylated starch-graft-poly acrylic acid (ASt-g-PAA).35

They also grafted AM on potato starch using the same three initiators and found that

the maximum grafting yield, grafting percentage and graft efficiency was obtained

with BPO.38

Scheme 5 Grafting of acrylic acid on hydrolyzed starch.37

Hydrogel based on grafting of L-aspartic acid on wheat starch was synthesized by

Vakili et al using two types of initiators, CAN and AIBN The maximum value of

grafting percentage for CAN and AIBN was 59.94 % and 80.25 %, respectively So

AIBN was found as better initiator than CAN in this case.39 Grafting of L-aspartic

acid on starch is given in Scheme 6

Scheme 6 Grafting of L-aspartic acid on starch.39

Wang et al prepared starch-graft-poly(2-methacryloyloxyethyl) trimethyl ammonium

chloride (St-g-PDMC) by grafting (2-methacryloyloxyethyl) trimethyl ammonium

chloride (DMC) on starch using KPS as a radical initiator A graft copolymer was

used for wastewater treatment.40 Tali et al also used KPS to graft AM and AA on

sorghum starch.41 Fakhru et al grafted MMA on starch using CAN and KPS separately The grafting percentage with CAN was 246 % and with KSP as the

initiator was 90 % So CAN was found better initiator than KPS The resultant

product may have application as a biodegradable plastic.31 Another important radical initiator is ammonium persulfate [(NH4)2S2O8] Song used this initiator for grafting

AM and acrylacyloxyethyltrimethyl ammonium chloride (AAC) on corn starch along

with urea as a co-initiator The products were found useful for waste water treatment

and gave better results than cationic polyacrylamide.42 Grafting of various monomers

on starch with different initiators is summarized in Table 1 Various grafting

parameters such as grafting percentage and grafting efficiency are also given in this

12

Table 1 Summary of grafting of various monomers on starch with different chemical initiators

efficiency

Grafting Percentage Reference(s)

ammonium chloride

Page 12 of 54 RSC Advances

2.1.1.2 Grafting induced by radiations

Fanta et al grafted vinyl acetate onto granular corn starch initiated by cobalt-60

irradiation The grafting efficiency of the grafted product was about 34 % The

optimum radiation dose was found to be 1.0 Mrad By the addition of smaller amount

of AM, MA and methacrylic acid (MMA) as comonomers the grafting efficiency was

increased and reached to 70 % when the concentration of MMA in monomers mixture

was increased to 10 % The grafting efficiency of 90 % was achieved when the

reaction was carried out near 0˚C.45 Sheikh et al grafted polystyrene (PST) on wheat

starch using gamma rays as radical initiators Maximum grafting yield (252.9 %) was

obtained when starch/styrene weight ratio was 1/3 and the applied dose was 10 kGy.46

Zhang et al synthesized St-g-PAM cross-linked with N, N-methyl bisacrylamide

(MBA) with 10 MeV electron beam irradiation at room temperature The optimum

dose was found to be 8 kGy, the optimum ratio of AM to AGU was 4.5 and the

optimum ratio of MBA to AM was 0.4 The resultant product showed excellent

absorbance and was categorized as superabsorbent polymer.47 Similarly, El-Mohdy et

al synthesized starch-graft-poly(ethylene glycol)-co-poly(methacrylic acid) PEG-co-PMAA) hydrogel from water soluble starch, ethylene glycol (EG) and

(St-g-methacrylic acid (MAA) using γ initiations as radical initiators.48 The synthetic route

Grafting induced by radiations is useful, because it requires less time than grafting

induced by chemicals thus prevents the waste of time

2.1.1.3 Grafting induced by enzymes

Chemical method of grafting has certain disadvantages such as difficulty in

controlling the reaction, impact of chemicals as a secondary pollutant and degradation

of starch

Thus, enzymatic grafting was found to be environmental friendly alternative for

classic chemical grafting Keeping in mind the importance of enzymatic grafting,

Wang et al used horseradish peroxidase (HRP) for grafting of poly(methyl acrylate)

(PMA) onto the soluble starch in the presence of hydrogen peroxide (H2O2) and acetyl acetone (Acac) as co-catalyst The grafting percentage and grafting efficiency under

optimal conditions were reached to 30.21 % and 45.13 %, respectively.49 The grafting

of PMA on starch is given in Scheme 8

Scheme 8 Enzyme catalyzed synthesis of St-g-PMA.49

With the same enzyme (horseradish peroxidase) Shogren et al grafted polyacrylamide

on maize starch in water using H2O2/2,4-pentanedione as co-catalyst.50 Similarly, Lv

et al grafted p-hydroxybenzoic acid (PHA) on corn starch with this system and the

resultant graft copolymer was found to have excellent tanning and retanning

properties The starch was first degraded with α-amylase and then treated further.51The degradation of starch followed by grafting of PHA on degraded starch is given in

Scheme 9 Using the same initiating system (horseradish peroxidase/H2O2), a new cationic starch has been prepared by grafting poly(dimethyldiallylammonium

chloride) (PDMDAAC) on starch The resultant product was used as a sludge

dewatering agent resulting in reduction of sludge water content to 50.6 % from 97.85

%.52 Grafting of PDMDAAC on starch is given in Scheme 10

Scheme 9 (a) Degradation of starch with α-amylase and (b) grafting of

p-hydroxybenzoic acid on starch with horseradish peroxidase.51

Scheme 10 Grafting of poly (dimethyl diallylammonium chloride) on starch.522.1.2 Grafting through living polymerization

In recent years, methods of ‘living polymerization’ have been developed to provide a

potential for grafting reactions The definition of living polymer is ‘that retains their

ability to propagate for a long time and grows to a desired maximum size while their

degree of termination or chain transfer is still negligible’.53 Conventional free-radical polymerization requires continuous initiation, with termination of the growing chain

radicals in coupling or disproportionation reactions, and as a result leads to unreactive

“dead” polymers and essentially time invariant degree of polymerization and broad

molecular weight distribution

In case of living polymerization, it provides living polymers with regulated molecular

weights and low polydispersities This method has got much interest because of its

well control over copolymer architecture Controlled free-radical polymerization may

be effective through atom transfer radical polymerization (ATRP).22 Wang et al

synthesized starch macro-initiator in 1-allyl-3-methylimidazolium chloride

([AMIM]Cl) by homogeneous esterification of starch with 2-bromoisobutyryl

bromide (BIBB) and then grafted PST and poly(methyl methacrylate) (PMMA) on

this macro-initiator through ATRP using CuBr/N,N,N′,N′,N′-pentamethyldiethylamine

(PMDETA) and CuBr/2,2-dipyridyl (BPY) as catalysts Compared with

heterogeneous grafting using traditional free radical initiators, grafting ratio was

greatly improved.54 Synthesis of macro-initiator and grafting of PST and PMMA on

this macro-initiator is given in Scheme 11 Bansal et al also grafted PST and PMMA

on expended starch using the same method as used by Wang et al.55 Liu et al

synthesized starch-graft- poly(n-butyl acrylate) St-g-PBA by surface initiated atom

transfer radical polymerization(SI-ATRP) of n-butyl acrylate (BA) with starch

bromo-acetic ester macro-initiator in the presence of 1,10-phenanthroline and Cu(I)Br as

catalyst in toluene The product was supposed to be used in preparation of the

biodegradable plastics.56 Synthesis of St-g-PBA is given in Scheme 12

Scheme 11 Synthesis of the corn starch-based ATRP macroinitiator and starch graft

Similarly, Wang et al prepared starch-graft-poly(N-isopropylacrylamide)

(St-g-PNIPAM) hydrogel by single electron transfer living radical polymerization

(SET-LRP) using starch-Br as a macro-initiator The resultant product was thermosensitive

with LCST range 31.5 ˚C to 23 ˚C varying with length of PNIPAM chains The

thermosensitivity was concluded from H1NMR The intensities of signals a, b, c and d,

which were the characteristic peaks of PNIPAM side chains, decreased obviously

during temperature change from 25 °C to 35 °C, indicating that St-g-PNIPAM have a

good response to temperature (Fig 2) The hydrogel was found to have good swelling

and rapid shrinking rate showing its application for drug delivery.57

Fig 2 1H NMR spectra of St-g-PNIPAM (a) at 25 ˚C and (b) at 35 ˚C in D2O (Adopted with permission from reference 57)

2.1.3 Ionic grafting

There are several examples in which starch has been grafted with acrylonitrile,

methacrylonitrile, acrylic and methacrylic esters and several other vinyl monomers in

the form of metal starch alkoxide Liquid ammonia, tetrahydrofuran,

N,N-dimethylformamide and dimethyl sulfoxide have been used as different solvent

systems.58 Tahan et al prepared starch-graft-poly(ethylene oxide) (St-g-PEO) in

DMSO 31.9 % to 63.9 % of the hydroxyl groups of starch were deprotonated forming

starch alkoxide ions However, the grafting yield was found to increase with the

increase in monomer concentration but independent of the alkoxide percentage.59Similarly, Houzé grafted poly(ε-caprolactone) (PCL) on granular starch in two-step

process In first step, hydroxyl groups were activated with alkyl aluminum derivatives

(AlEt3) In the second step, ε-caprolactone (CL) was grafted onto starch with ring opening polymerization (ROP) The two step procedure of grafting of PCL on starch

is given in Scheme 13

Scheme 13 Two-step procedure for the in situ polymerization of ε-caprolactone from

the starch granule.60

They also grafted poly(δ-valerolactone) on starch in similar way as PCL.60 Phenyl glycidyl ether has also been grafted on starch in DMSO.61 Cohen et al prepared

starch grafted with poly(lauryl methacrylate) in DMSO using potassium alkoxides

derivative of starch It was found that the grafting yield increased with the increase in

alkoxide concentration However with increase in monomer concentration and rise in

temperature the homopolymerization increased.62 Under the same conditions the anionic grafting of methyl methacrylate with starch alkoxide was also studied and it

was found that grafting yield was directly related with alkoxide concentration and

inversely related with temperature and monomer concentration.63

2.2 Cross-linking

Cross-link is the chemical bond that links one polymer chain to another and this

phenomenon of making link between two polymer chains is called

cross-linking Epichlorohydrin (EPI) is the most familiar cross-linking agent which is used

in cross-linking of polysaccharides Jyothi et al cross-linked cassava starch by EPI in

three different media including water with phase transfer catalyst (PTC), water without phase transfer catalyst and N,N-dimethylformamide (DMF)

Tetrabutylammonium bromide (TBAB) was used as PTC The highest degree of cross-linking was obtained, when the reaction was carried out in DMF The modified starch had higher water-binding capacities (WBC) and their α-amylase digestibility was found to decrease with the increase in their degree of cross-linking.64 Guo prepared cross-linked porous starch (CPS) by cross-linking corn starch with EPI and then hydrolyzing it with α-amylase This CPS was found to be biodegradable and safe adsorbent having higher adsorption capability than native starch This porous starch

N,N′-methylenebis(acrylamide) (MBAA) is another excellent cross-linking agent Hu et al

enzymolysed waxy corn starch and then cross-linked this starch with MBAA, in the presence of CAN for chromium (VI) adsorption The resultant CPS also had excellent adsorption capacity for the other heavy metal ions like cadmium (II) ion and lead (II) ion.66 POCl3 is another interesting cross linking agent Singh et al cross-linked sago

starch with POCl3 which resulted in a highly substituted cross-linked starch phosphate having higher thermal stability and swelling behavior.67 Kim et al used POCl3 for cross-linking corn starch at different pressure from 0.1 MPa to 400 MPa in order to

determine the effect of ultra-high pressure on the extent of cross-linking The increase

in cross-linking with pressure was revealed by the decrease in swelling and

gelatinization.68 Sodium trimetaphosphate (STMP) is another well-known

cross-linking agent Hong et al cross-linked granular maize starch with STMP in four

different reaction media (deionized water, aqueous sodium sulfate solution, aqueous

ethanol and aqueous acetone) in order to check the effect of various reaction media on

the degree of cross-linking The degree of cross-linking of starch in aqueous ethanol

and aqueous acetone was higher than in deionized water and aqueous sodium sulfate

solution.69 Carbinatto et al used STMP for cross-linking pectin–high amylose starch

mixtures with different ratio (1:4, 1:1 and 4:1) Cross-linked samples were found to

have higher thermal stabilities The sample having higher amylose content showed

higher cross-linking.70 Wongsagonsup et al cross-linked tapioca starch using mixture

of STMP and sodium tripolyphosphate (STPP) (99:1 (w/w) ratio) When STMP was

used as cross-linking agent , phosphorous content was about 0.04 %, and when

STMP and STPP were used as cross-linking agents, the content of phosphorous was

increased by 10 times.71 This shows that the mixture of STMP and STPP is better cross-linking system than STMP Citric acid is another important cross-linking agent

Reddy et al cross-linked starch films with citric acid to improve their tensile strength

and thermal stability and to decrease their dissolution in water and formic acid The

resultant cross-linked films had 150 % higher tensile strength than normal films.72

The whole process of cross-linking is summarized in Table 2

Table 2 Cross-linking of different starches with different cross-linkers

1 Cassava

starch, porous corn starch

69-71

2.3 Esterification of starch

Esterification of starch can be carried out with acids and their derivatives due to the

presence of large number of hydroxyl groups in starch To obtain product with high

degree of substitution, the reaction should be carried out in organic solvent.21 Fang esterified four different types of starches, each having different ratio of amylose and

amylopectin with acid chlorides of different chain length However, esterification

occurred with acid chlorides which contained 6-8 carbon atoms but not with acid

chlorides which had carbon atoms less than 6 or more than 8 because in such cases

hydrolysis (reverse reaction) was dominant over esterification The maximum degree

of substitution obtained was almost 3.73 Similarly, Chi et al acetylated corn starch

with acetic anhydride as acetylating agent Different degrees of substitution (DS)

(0.85, 1.78 and 2.89) were obtained under different temperature conditions (50 ˚C, 65

˚C and 75 ˚C, respectively).74 Mei et al found that when cassava starch was esterified

with citric acid, degree of substitution was increased from 0.058 to 0.178 with the

increase in citric acid concentration from 10 % to 30 % However on further increase

in concentration of citric acid to 40 % the DS value was decreased to 0.129 The

starch citrate was found to have lower swelling power and solubility than native

starch showing the increase in resistant starch content in the starch sample with

esterification.75 Potato starch oleate ester was synthesized in methylimidazolium chloride reaction medium using immobilized lipase as a catalyst

1-butyl-3-at different time and temper1-butyl-3-ature The maximum DS (0.22) was obtained when

reaction was carried out at 60 °C for 4 hours The product could be used for

biodegradable packaging and as carrier for bioactive agents.76 Corn starch was esterified with malic anhydride and its composite with polylactic acid (PLA) was

prepared It was found that the tensile strength and bending strength of esterified

starch (ES)/PLA composite were higher than those of the native starch (NS)/PLA

composite.77 Lipase-coupling esterification of waxy corn starch was carried out with octenyl succinic anhydride and it was found that the DS value of 0.0195 and the

reaction efficiency of 84.05 ± 2.07 % could be obtained in 30 minutes The reduction

in reaction time was found to be useful for producing the product on large scale in

industry.78 Starch betainate, a cationic starch derivative, was prepared by esterification of starch with betainyl chloride (BC) BC was first prepared from

anhydrous betaine and thionyl chloride and then the esterification process was carried

out The product was found to greatly increase the strength of paper.79 The schematic

summarization of esterification is given in Scheme 14

Scheme 14 Schematic summary of esterification.73-79

2.4 Etherification of starch

Etherification usually results in four types of the modified products, non-ionic,

cationic, anionic and amphoteric products On the basis of product obtained,

etherification is divided into four types.21

2.4.1 Non-ionic etherification

Huijbrechts et al etherified waxy maize starch and high amylose maize starch with

allyl glycidyl ether to give 1-allyloxy-2-hydroxy-propyl starches with DS of 0.19 ± 1

and 0.20 ± 0.01, respectively The reaction was regioselective and occurred mostly at

carbon 6 of anhydroglucose unit of starch.80 Azo and anthraquinone dyes, which are very toxic and cancer causing,81 can be removed with tertiary amine starch ether, ( 2,4-bis(dimethyl amino)-[1,3,5]-triazine-6-yl ) starch (BDATS) This modified starch

was synthesized by shi et al by etherification of normal starch with 2,4-bis(dimethyl

amino)-6-chloro-[1,3,5] triazine (BDAT).82 The flocculation behavior of this starch

with dye is given in Scheme 15

Scheme 15 The flocculating behavior BDATS.82

In order to improve compatibility of polylactide(PLA)/starch composite, Wokadala et

al etherified waxy and amylose-enriched starches with 1,2-epoxybutane which

resulted in the products with DS of 2.0 and 2.1, respectively The

PLA/butyl-etherified waxy and high amylose starch composite films were found to be more

flexible and had higher elongation at break compared to PLA/non-butyl-etherified

composite films.83

Misman et al etherified sago starch with benzyl chloride in water and in 70 % ethanol

and found that solvent (ethanol) based etherification resulted in product with the

higher DS, higher thermal stability and better flow ability.84 Simillarly when high amylose corn starch (HACS) was etherified with 1-bromopropane, the etherified

and its biodegradation rate was found to decrease with increase in degree of

etherification.85

2.4.2 Cationic etherification

Cationic starches are produced by the reaction of starches with reagents containing

amino, imino, ammonium, sulphonium, or phosphonium groups.1, 86 Pal et al

synthesized a cationic starch by etherifying cationic moiety

N-(3-chloro-2-hydroxypropyl) trimethyl ammonium chloride (CHPTAC) with starch and found it to

be an effective flocculating agent for the treatment of wastewater.87 Kavaliauskaite et

al synthesized cationic starch and cross-linked cationic starch with DS 0.2–0.85 and

reaction efficiency 82–93 % by esterification of normal starch and cross-linked starch

with glycidyltrimethylammonium chloride in heterogeneous conditions.88 Jiang et al

etherified corn starch with cationic moiety, hydroxymethyl dimethylamine

hydrochloride (HMMAHC) which resulted in a cationic starch ether, starch–

methylene dimethylamine hydrochloride (SMMAHC) by Mannich reaction The

product was found as an effective flocculant Because of the hydrolysis of the product

and etherifying reagent in water, dry method was proposed for better yield.89 Heinze

et al prepared water soluble cationic starch by etherifying starch with

2,3-epoxypropyltrimethyl-ammonium chloride in different reaction media i.e water,

dimethyl sulfoxide and ethanol/water mixture When reaction was carried out in

water, highest grafting efficiency was observed.90 Similarly, Wei et al prepared

different cationic starch derivatives by etherifying glycidyl octyldimethylammonium

chloride (GODAC), glycidyl dodecyl dimethylammonium chloride (GDDAC) and

glycidyl tetradecyl dimethylammonium chloride (GTDAC) onto starch by wet

(solvent), kneading and microwave-assisted methods However, the starch derivatives,

obtained by microwave-assisted method was found to have higher flocculating

efficiency than obtained by the other methods.91

2.4.3 Anionic etherification

Anionic starches obtained by anionic etherification were found to be useful

flocculants in wastewater treatment Carboxymethyl starch (CMS), carboxyethyl

starch, starch-2-hydroxypropylphosphate and starch-2-hydroxypropylsulfate have

been prepared and applied as flocculants.92 The most important anionic starch is

carboxymethyl starch Yanli et al synthesized carboxymethyl yam starch by the

reaction of starch with monochloroacetic acid (MCA) in the presence of sodium

hydroxide.93 Similarly Wang et al synthesized carboxymethyl kudzu root starch by

the same procedure.94 Etherified starch with anionic moiety can be used for the removal of heavy metals and basic dyes from wastewater

2.4.4 Amphoteric etherification

Amphoteric starches having both cationic and anionic functional groups have been

prepared and these starches were found as excellent flocculating agents and showed

good adsorbing properties Generally, the anionic groups used in amphoteric starches

are phosphonate, phosphate, sulfonate, sulfate and carboxyl groups, while the cationic

groups are quaternary ammonium and tertiary amino groups.21 Lin et al etherified

waxy maiz starch with cationic moiety, 2,3-epoxypropyl trimethyl ammonium

chloride, and anionic moiety, phosphate to give amphoteric copolymer which had

excellent flocculating and heavy metal adsorption capacity So it could be used for

wastewater treatment.95 With amphoteric starch we can remove cationic anionic impurities simultaneously The summary of etherification, using different conditions