Chemical modification of starch and its application as an adsorbent material

Abstract

Starch is a biopolymer of 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 from water.

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1 Introduction

The 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% α(1–6) and 95% α(1–4) linkages. Small amount of proteins and lipids are also present in starch.^2,3

Naturally 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 environmental managers.⁸

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.¹³ Dithiocarbamate 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 supposed to form chelation and ionic interactions with heavy metals causing the 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 Cu²⁺, Pb²⁺ and Cd²⁺.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.¹⁹

The 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 polymerization path.

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 (Fe²⁺ + 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⁻¹ 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 Ce⁴⁺.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 Fe²⁺/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 (Fe²⁺ + 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 H₂SO₄ for grafting AM on starch. With this system, grafting yield and grafting efficiency were increased and the homopolymer content was decreased in comparison with KMnO₄ alone.³⁶ Djordjevic et al. found that when AA was grafted onto the hydrolyzed potato starch in the presence of three different type of initiators i.e. AIBN, 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

Scheme 5 Grafting of acrylic acid on hydrolyzed starch.³⁷

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

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 table.

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

S/no.	Starch	Monomer(s)	Initiator(s)	Grafting efficiency	Grafting Percentage	Reference(s)
1	Cassava	Methacrylate	CAN	46.3	79.9	27
2	Potato	Acrylic acid	CAN	30.63	61.25	28
3	Potato/cassava	Acrylic acid	Fe^2+/H2O2	44.1	—	33 and 35
4	Potato	Acrylonitrile	CAN/Ce(SO4)2	—	218.38	29
5	Potato	Methyl acrylate	CAN	—	—	43
6	Potato	Acrylamide	KPS	69.85	30.25	38
7	Potato	Acrylamide	AIBN	78.09	30.31	38
8	Potato	Acrylamide	BPO	93.37	36.96	38
9	Corn	Acrylamide	KMnO4/HIO4/H2SO4	93	90	36
10	Cassava	Acrylamide	CAN	—	174.8	32
11	Maize	Acrylamide	CAN/microwave	—	907	30
12	Sago	Methyl methacrylate	KPS	—	90	31
13	Sago	Methyl methacrylate	CAN	—	246	31
14	Potato	Phenyl methacrylate	KPS	76	43.2	44
15	Corn	Acrylamide/acrylacyloxyethyltrimethyl ammonium chloride	CO[(NH2)2]/[(NH4)S2O8]	—	215	42
16	Wheat	L-Aspartic acid	CAN	43.32	54.94	39
17	Wheat	L-Aspartic acid	AIBN	62.57	80.25	39

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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) (St-g-PEG-co-PMAA) hydrogel from water soluble starch, ethylene glycol (EG) and methacrylic acid (MAA) using γ initiations as radical initiators.⁴⁸ The synthetic route is given in Scheme 7.

Scheme 7 Synthesis of St-g-PEG-co-PMAA hydrogel.⁴⁸

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.⁵¹ The 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.⁵²

2.1.2 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 copolymers.⁵⁴

Scheme 12 Preparation route of St-g-PBA.⁵⁶

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 ¹H NMR. 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 ref. 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.⁵⁹ Similarly, 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 cross-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 was applied to remove methylene blue (MB) from water.⁶⁵ 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

S/no.	Starch	Cross-linker	Product	Reference(s)
1	Cassava starch, porous corn starch			64 and 65
2	Waxy corn starch			66
3	Sago starch, corn starch			67 and 68
4	Granular maize starch, pectin-high amylose starch mixture, tapioca starch			69–71
5	Corn starch			72

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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 1-butyl-3-methylimidazolium chloride reaction medium using immobilized lipase as a catalyst at different time and temperature. 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 Similarly when high amylose corn starch (HACS) was etherified with 1-bromopropane, the etherified HACS was found to have higher decomposition temperature than unmodified HACS 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 maize 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 is given in Table 3.

Table 3 Summary of etherification of starch along with reaction parameters

S/no.	Type of etherification	Reaction with experimental details	Reference(s)
1	Non-ionic etherification		80
2	Non-ionic etherification		83
3	Non-ionic etherification		84
4	Non-ionic etherification		85
5	Cationic etherification		87 and 90
6	Cationic etherification		88 and 90
7	Cationic etherification		89
8	Cationic etherification		91
9	Anionic etherification		92–94
10	Anionic etherification		92
11	Amphoteric etherification		95

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2.5 Dual modification

Sometimes single modification does not impart the required properties to the starch. Therefore, it is modified further to obtain the product with the required properties, so that the modified starch may be used in industry.²¹ Dual modification may be physical–chemical modification,⁹⁶ physical–enzymatic modification,⁹⁷ chemical–enzymatic modification⁹⁸ and chemical–chemical modification^99–101 but the most important dual modification is chemical–chemical modification, which involves two types of chemical modifications (acetylation/oxidation, cross-linking/acetylation, or cross-linking/hydroxypropylation). Starch with such type of modification has lot of applications in food and non-food industries (emulsifiers, binders and thickeners) but the most important application of such starch is its use as an adsorbent material in wastewater treatment.¹ In order to check the change in physiochemical properties with chemical modification, Sukhija et al. carried out dual modification of elephant foot yam starch on two different ways to get two types of modified starches i.e. oxidized cross-linked starch (OCS) and cross-linked oxidized starch (COS). OCS was prepared by oxidation of starch with sodium hypochlorite (NaOCl) followed by cross-linking with STMP while COS was prepared by cross-linking of starch with STMP followed by oxidation with NaOCl. OCS was found to have improved paste clarity, solubility, and thermal characteristics as compared to COS.¹⁰² The oxidation and cross-linking reactions of starch are given in Scheme 16. Zhao et al. cross-linked, hydroxypropylated and dual modified sweet potato starch of different granule size and found that large-sized granules underwent modification more easily than smaller ones.¹⁰³ Mehboob et al. modified sorgosam starch by succinylation, acid-thinning and combination of these two process in order to check the effect of these modifications on the properties of starch. Succinylation of both native starch and acid-thinned starches decreased their retrogradation and increased their setback viscosity.¹⁰⁴ Similarly, to check the effect of single and dual modification of starch on its properties, Lee et al. modified japonica (JR) and indica (IR) rice starches by acetylation, hydroxypropylation, cross-linking and dual modification (cross-linking followed by acetylation and cross-linking followed by hydroxypropylation). Dually modified starches (cross-linked acylated and cross-linked propylated starch) were found to have increased gelatinization temperature, gel hardness and gel chewiness and decreased breakdown, swelling and solubility.¹⁰⁵ When Liu et al. prepared cross-linked starch, oxidized starch and COS using hydrogen peroxide as oxidizing agent and sodium STMP as a cross-linker, COS was found to have better retrogradation properties and higher hydrophilic properties than the other starches.¹⁰⁶

Scheme 16 Chemical reaction: (a) sodium hypochlorite (NaOCl) oxidation of starch and (b) sodium trimetaphosphate (STMP) cross-linking of starch.¹⁰²

When Woggum et al. modified rice starch by hydroxypropylation with propylene oxide followed by crosslinking with STMP, it was found that with the increase in propylene oxide concentration, constant decrease in the pasting temperature, paste consistency, setback and gel strength were observed.¹⁰⁷ Kittipongpatana et al. cross-linked carboxymethyl rice starch with different concentration of EPI and found that using 5–7.5% of EPI resulted in the increase in resistant starch content, but when 1–3% of EPI was used, the resultant product indicated good swelling and water absorbing properties showing its potential application as water-absorbent tablet disintegrant.¹⁰⁸ Flores et al. modified banana starch with oxidation followed by acetylation and used modified starches for film formation. Films prepared with the double-modified banana starch were found to have better physical, mechanical and barrier properties than those made from acetylated starch.¹⁰⁹ Sweet potato starch was acetylated and dual modified (oxidized with propylene oxide and cross-linked by adipic acid) and it was found that both type of starches had improved water binding and oil-binding capacities, solubility, paste clarity and gel strength.¹¹⁰

3 Application of the modified starch as an adsorbent

Application of modified starch as an adsorbent can be categorized in two groups.

3.1 Removal of heavy metals

Heavy metals are unfit for human beings and aquatic life and cause diseases like cancer, hyperkeratosis, tremor and depression in human beings⁵ and cardiovascular hematologic reproductive metabolic and endocrine disturbances and necrosis in fish.⁶ So their removal is very necessary.

Dong et al. used amino starch to remove Cu²⁺ and Cr⁴⁺ from water. The adsorption ability was higher for Cu²⁺ than Cr⁴⁺.¹¹¹ Kweon et al. used succinylated corn starch and oxidized corn starch for the adsorption of Cu²⁺, Zn²⁺, Pb²⁺ and Cd²⁺. Succinylated starch was more effective than oxidized starch.¹¹² Kim et al. used carboxymethyl cross-linked starch to remove divalent toxic cations (Cu²⁺, Pb²⁺, Cd²⁺, and Hg²⁺) from wastewater.¹¹³ Güçlü et al. used starch-graft-acrylic acid/montmorillonite (St-g-AA/MMT) nanocomposite hydrogels for the removal of Cu²⁺ and Pb²⁺ from aqueous solutions. Maximum adsorption was observed at pH 4.¹¹⁴ Keleş et al. used St-g-AA for the removal of Pb²⁺, Cu²⁺ and Cd²⁺ from aqueous solution. The removal of these ions was in the following sequence: Pb²⁺ > Cu²⁺ > Cd²⁺.¹¹⁵ Xu et al. applied cross-linked amphoteric starch having quaternary ammonium and carboxymethyl groups for adsorption of Pb²⁺ from water. The maximum adsorption occurred at pH 4–5.¹¹⁶ They also used such starch to remove Cr⁴⁺ from water.¹¹⁷ Sancey et al. used cross-linked carboxymethyl corn starch for adsorption of heavy metals from industrial effluents. Cu²⁺ and Fe²⁺ were completely removed, while concentration of Pb²⁺, Cd²⁺ and Ni²⁺ was greatly decreased and concentration of Zn²⁺ was decreased to the legally permitted limit.¹¹⁸ Guo et al. used cross-linked starch phosphate carbamates (CSPC) for the adsorption of Pb²⁺ from wastewater. The adsorption process was endothermic.¹¹⁹ Parvathy et al. used cassava starch-graft-polyacrylamide hydrogel to remove heavy metals from wastewater. The removal capacity of this modified starch for different heavy metals was found in the following sequence: Cu²⁺ > Pb²⁺ > Zn²⁺.³² Ding et al. used dialdehyde 8-aminoquinoline starch (DASQA) to adsorb Zn²⁺ ions from water.¹²⁰ Xiang et al. used DTCS (normal, porous and enzymolysed) for adsorption of heavy metals.¹⁴ The mechanism of the adsorption of Cu²⁺ by dithiocarbamate starch is given in Scheme 17.

Scheme 17 Proposed mechanism of Cu²⁺ adsorption on the DTC modified starch derivatives.¹⁴

Liu et al. used dialdehyde 5-aminophenanthroline starch (DASAPL) to remove Cd²⁺ from water. The adsorption phenomenon was exothermic and obeyed Freundlich and Langmuir isothermal model.¹²¹ Zheng et al. used starch-graft-poly(acrylic acid)/sodium humate (St-g-PAA/SH) hydrogels for the adsorption of Cu²⁺. The adsorption occurred by the ion exchange and chelation between carboxylic acid and Cu²⁺ ions and it was confirmed by pH and FTIR studies.¹²² When Li et al. used cross-linked amino starch (CAS) and DTCS to remove Cu²⁺ from water, they found that DTCS had higher adsorption ability than CAS for Cu²⁺.¹²³ Khalil et al. used saponified St-g-PMA for the removal of different heavy metal ions from water. The adsorption was found in the following order: Hg²⁺ > Cu²⁺ > Zn²⁺ > Pb²⁺.¹²⁴ Kolya et al. used hydroxyethyl starch-graft-poly(N,N-dimethyl acrylamide) (HES-g-PDMA) and hydroxyethyl starch-graft-polyacrylamide (HES-g-PAM) to remove heavy metals from water. The adsorption capacity of these two substrates was found in the following order: Hg²⁺ > Cu²⁺ > Zn²⁺ > Ni²⁺ > Pb²⁺. In these two copolymers, the adsorption performance of HES-g-PAM was better than HES-g-PDMA.¹²⁵ Fig. 3 shows the metal ion removal capacity. They also used starch-graft-poly(N,N-dimethylacrylamide)-co-acrylic acid (St-g-PDMA-co-PAA) adsorbent for adsorption of Cr⁴⁺ from water.

Fig. 3 Comparison of the metal ion removal capacity of HES-g-PAM and HES-g-PDMA under optimum conditions (adopted with permission from ref. 125).

Cheng et al. found that dithiocarbamate-modified glycidyl methacrylate starch (DMGS), a multidentate ligand removed heavy metals by chelation process. The removal sequence of different metal ions was in the order: Cu²⁺ > Cd²⁺ > Co²⁺ > Zn²⁺ > Ni²⁺ > Mn²⁺.¹²⁶ Xie et al. used amino modified starch (AMS) to remove Cd²⁺ from aqueous solution. The process was endothermic, spontaneous and followed Langmuir isothermal model.¹²⁷ Chang et al. used cross-linked starch-graft-polyacrylamide-co-sodium xanthate to remove Cu²⁺ from water and it was found better adsorbent than cross-linked starch xanthate and cross-linked starch-graft-polyacrylamide.¹²⁸ Irani et al. used polyethylene-graft-poly(acrylic acid)-co-starch/organo-montmorillonite hydrogel composite to removed Pb²⁺ from the water. Fig. 4 shows the morphological changes in the hydrogel composite before and after Pb²⁺ adsorption. The surface of the hydrogel composite is rough before adsorption of Pb²⁺ and after adsorption of Pb²⁺ the surface becomes smooth.¹²⁹

Fig. 4 SEM images of hydrogel composites: (a) prior to Pb²⁺ adsorption and (b) after Pb²⁺ adsorption (adopted with permission from ref. 129).

Adsorption of different heavy metals by different starch derivatives, along with the maximum adsorption efficiencies of these starch derivatives for specific heavy metals, are given in Table 4.

Table 4 Adsorption of heavy metals by different starch derivatives with maximum adsorption efficiencies

S/no.	Type of starch derivatives	Heavy metals adsorbed	Adsorption efficiency (mg g^−1)	Reference(s)
1	Amino starch, cross-linked amino starch	Cu^2+	136.01	66,111,123 and 127
		Cr^4+	22.8
2	Succinylated starch	Pb^2+	110.64	112 and 130
		Cu^2+	32.79
		Zn^2+	13.47
		Cd^2+	12.36
3	Oxidized starch	Cu^2+	79.114	112
		Pb^2+	48.27
		Zn^2+	37.27
		Cd^2+	14.61
4	Carboxymethyl cross-linked starch	Cu^2+	—	113
		Pb^2+	—
		Cd^2+	—
		Hg^2+	—
5	Starch-graft-polyacrylic acid	Pb^2+	112.96	115,131,132 and 133
		Cu^2+	60.36
		Cd^2+	588
		Co^2+	14.73
		Ni^2+	12.91
		Zn^2+	13.07
6	Cross-linked amphoteric starch	Pb^2+	152.74	116,117 and 134
		Cr^4+	30.68
		Cu^2+	84.4
7	Cross-linked starch phosphate carbamates	Pb^2+	316.47	119
8	Starch-graft-polyacrylamide	Cu^2+	—	32
		Pb^2+	—
		Zn^2+	—
9	Dialdehyde 8-aminoquinoline starch	Zn^2+	77.91	120
10	Starch esters	Zn^2+	8.44	16
		Cd^2+	7.54
		Ni^2+	6.38
		Pb^2+	25.16
11	Dithiocarbamate-modified starch	Cu^2+	66.09	14 and 123
12	Dialdehyde 5-aminophenanthroline starch	Cd^2+	1124.11	121
13	Hydroxyethyl starch-graft-poly(N,N-dimethyl acrylamide)	Hg^2+	461.357	125
		Cu^2+	—
		Zn^2+	—
		Ni^2+	—
		Pb^2+	103.6
14	Hydroxyethyl starch-graft-polyacrylamide	Hg^2+	300.88	125
		Cu^2+	—
		Zn^2+	—
		Ni^2+	—
		Pb^2+	20.72
15	Starch-graft-poly(N,N-dimethylacrylamide)-co-acrylic acid	Cr^4+	6.70	135
16	Starch-graft-polyacrylamide-co-sodium xanthate	Cu^2+	—	136 and 128
		Ni^2+	—
17	Dithiocarbamate-modified glycidyl methacrylate starch	Cu^2+	20.33	126
		Cd^2+	28.21
		Co^2+	13.55
		Zn^2+	13.33
		Ni^2+	10.74
		Mn^2+	7.47
18	Polyethylene-graft-poly(acrylic acid)-co-starch/organo-montmorillonite hydrogel composite	Pb^2+	430	129

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3.2 Removal of dyes

Discharge of dyes into water is a big environmental issue. The various sources of dyes are human activities, plastics, textiles, papers and cosmetic industries.^137–139 The dye contaminant discharge is more than 70 000 tons per annum.¹⁴⁰ The dyes should be removed from water because these resist to degradation, cause allergies and are mutagenic.¹⁴¹ In human beings, dyes cause kidney disorder, defect reproductive system and damage central nervous system, liver and brain.¹⁴²

Various biological and physico-chemical methods have been used for the removal of these dyes from water but the most important and effective one is the removal of these dyes by adsorption with low cost biodegradable substances^143,144 like modified starch. Sekhavat et al. synthesized magnetic nanocomposite hydrogel (m-CVP) beads 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 and used these beads for the removal of congo red (CR) and crystal violet (CV) dyes along with some transition metal ions like Cu²⁺, Pb²⁺ and Cd²⁺. Chemisorption occurred spontaneously and endothermically and fitted with Langmuir model. The important point related to the beads was their reusability. The reusability chat of these beads is shown in Fig. 5(a). Another important thing was ease of separation of these beads by external magnet, which prevented secondary pollution in water (Fig. 5 (b)).¹⁸ Xu et al. used cross-linked amphoteric starch having quaternary ammonium and carboxymethyl groups for the removal of acid dyes (acid light yellow 2G, acid red G) and basic dyes (methyl green, methyl violet). It was found that the acid dyes were removed by ammonium group and the basic dyes were removed by the carboxymethyl group. The optimum pH for the acid dyes was 2.16 and for basic dyes was 7.¹⁹

Fig. 5 (a) Reusability of m-CVP beads during four cycles and (b) m-CVP bead separation by an external magnetic field (adopted with permission from ref. 18).

Cheng et al. used DTCS to adsorb anionic dyes from water. The process was exothermic and spontaneous. The adsorption ability was in the following order: acid orange 7 > acid orange 10 > acid red 18 > acid black 1 > acid green 25 which was in inverse order of their molecular size. The adsorption was pH dependent and decreased with the increase in pH.¹⁴⁵ They also used dithiocarbamate-modified starch complexed with Ni²⁺ (DTCSNi) for the removal of these dyes and it was found that the adsorption property of such complex was also decreased with increase in pH, which might be due to the formation of more hydroxyl groups at higher pH and such hydroxyl groups competed with sulphate group of dyes, thus preventing its chelation with Ni²⁺. The adsorption sequence was in the following order: acid orange 7 > acid green 25 > acid red 18 > acid orange 10.¹⁴⁶ Similarly Wang et al. compared cross-linked amino starch and cross-linked amino enzymolysed starch for the adsorption of different dyes (acid orange 7, acid orange 10, acid green 25, and acid red 18). Cross-linked amino enzymolysed starch showed higher adsorption efficiency than cross-linked amino starch. In both cases, the forces responsible for the adsorption were electrostatic interaction and hydrogen bonding as shown in Scheme 18.¹⁴⁷

Scheme 18 Interaction models of (a) acid green 25 and (b) acid orange 7 on cross-linked amino enzymolysed starch.¹⁴⁷

Cheng et al. used similar amino starch for comparative removal of acid orange 10, acid green 25 and amido black 10B from water. The sequence of removal of these dyes was found in the order: amido black 10B > acid green 25 > acid orange 10.¹⁴⁸ Shimei et al. removed basic green 4 by the amphoteric starch having positive ammonium group and negative carboxymethyl group. The removal percentage was directly related with DS of carboxymethyl group. This process was exothermic, spontaneous and followed Freundlich isotherm and pseudo second order kinetic.¹⁴⁹ Gomes et al. used nanocomposites of St-g-PAA and cellulose nanowhiskers (CNWs) for the removal of methylene blue (MB) from water. The interactions responsible for the adsorption of MB on these composites were ionic bonding, hydrogen bonding and hydrophobic interaction.¹⁵⁰ Chen et al. also removed MB with composite hydrogel beads of starch and humic acid. The adsorption occurred by the interactions between MB and the anionic and aromatic groups of the humic acid. The process was chemisorption, spontaneous and was in accordance with pseudo second order kinetic. It was noted that the composite could be regenerated and reused for many cycles.¹⁵¹ Hebeish et al. removed MB (100%) from water using specific concentration of starch hydroxypropyl sulphate for specific time.¹⁵² Kolya et al. removed malachite green from water by hydroxyethyl starch-graft-poly(N,N-dimethyl acrylamide)-co-acrylic acid. It was found that adsorption was increased with increase in pH. The reason was that at low pH, amide group and dye both got protonated and became positively charged due to which they repelled each other and less amount of adsorption occurred. At higher pH, amide group was found to remain as such, but the carboxylic acid group deprotonated and electrostatic interaction was generated between anionic carboxylate group of the modified starch and cationic amine group of the dye. The maximum adsorption was observed at pH 5.5.¹⁵³ Halim found that maize starch grafted with poly(N,N-diethylaminoethyl methacrylate) and cross-linked with EPI showed excellent adsorbance for anionic dye like direct red 81 at low pH. Almost all dye (95.65%) was removed at pH 1. The adsorption was continuously decreased with the increase in pH until it reached to the zero at pH 10. The graph is shown in Fig. 6. On the basis of pH studies, it was found that interaction responsible for the adsorption of this dye was ionic interaction between protonated amino group of modified starch and sulfonate group of anionic dye.¹⁵⁴

Fig. 6 Effect of adsorbate medium pH on the percent dye removal (adopted with permission from ref. 154).

Güçlü et al. removed a basic dye, safranine T from aqueous solution by St-g-AA. The adsorption process was found to increase with rise in pH and followed Freundlich isotherm.¹⁵⁵ Al et al. used St-g-PAA/Na-montmorillonite for the adsorption of this dye and found that adsorption process followed the same Freundlich isotherm.¹⁵⁶ Klimaviciute et al. removed different anionic dyes like acid yellow 36, acid blue 78, acid orange 7, acid orange 52, acid blue 25 and acid red 151 from water using cross-linked starch with quaternary ammonium group and non-cross-linked starch with quaternary ammonium group. For both type of starches, the adsorption model was fitted with Langmuir model and the mechanism of adsorption was ionic interaction between positively charged ammonium groups of starch and negatively charged anionic dyes. When adsorption efficiency of both types of starches were compared, it was found that non-cross-linked starch showed less adsorption than cross-linked starch because of solubility problem.¹⁵⁷ The structure of the dyes discussed above are given in Scheme 19 and the brief summary of the adsorption of dyes by different starch derivatives along with their maximum adsorption efficiencies for particular dye is given in Table 5.

Scheme 19 Structure of dyes (a) acid red G, (b) acid light yellow 2G, (c) methyl violet, (d) safranine T, (e) methyl green, (f) methylene blue, (g) acid orange 10, (h) acid orange 7, (i) acid orange 52, (j) acid green 25, (k) congo red, (l) acid black 1, (m) acid red 18, (n) acid blue 78, (o) acid blue 25, (p) reactive blue, (q) acid yellow 36 and (r) malachite green.^{19,20,145–157}

Table 5 Adsorption of dyes with different starch derivatives

S/no.	Type of starch derivatives	Dyes adsorbed	Adsorption efficiency (mg g^−1)	Reference(s)
1	Cross-linked amphoteric starch	Acid light yellow 2G	227.27	19 and 149
		Acid red G	217.39
		Methyl green	133.33
		Methyl violet	333.33
		Basic green 4	104.75
2	Dithiocarbamate-modified starch	Acid orange 7	281.30	145
		Acid orange 10	196.32
		Acid red 18	149.41
		Acid black 1	219.30
		Acid green 25	245.42
3	Cross-linked amino starch	Acid orange 7	883.15	147 and 148
		Acid orange 10	561.84
		Acid green 25	831.70
		Acid red 18	949.03
		Amido black 10B	650.39
4	Starch/humic acid composite	Methylene blue	111.10	151
5	Hydroxyethyl starch-graft-poly(N,N-dimethyl acrylamide)-co-acrylic acid	Malachite green	—	153
6	Cross-linked starch-g-poly(N,N-diethylaminoethyl methacrylate)	Direct red 81	112	154
7	Starch-graft-polyacrylic acid	Safranine T	204	155
8	Starch-graft-polyacrylic acid/Na-montmorillonite composite	Safranine T	2237	156
9	Cationic starch with quaternary ammonium group	Acid yellow 36	—	157
		Acid blue 78	—
		Acid orange 7	—
		Acid orange 52	—
		Acid blue 25	—
		Acid red 151	—

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4 Conclusion

Starch is a natural biodegradable biopolymer of the plant origin which is modified by various methods to improve its useful properties and eradicate its negative impacts, which limit its use. Among various techniques used for the modification of starch, chemical modification is the most important and advance technique which mainly includes grafting, cross-linking, esterification, etherification and dual modification. The modified starch has many applications, but the most important one is its use as an adsorbent material for the removal of heavy metals and organic dyes from wastewater as their presence in water is not only harmful for aquatic life but also effect human life. Adsorbents other than modified starch are either costly or unable to regenerate. So, modified starch is considered as prominent adsorbent for the removal of these materials from wastewater.

However, there are still a lot of possibilities to improve its adsorption ability for heavy metals and organic dyes. We can use enzymolysed starch and porous starch instead of normal starch to improve its adsorption efficiency. The solubility problems in certain modified starch must be traced and minimized for better reusability. Starch may also be modified for adsorption of CO₂, SO₂ and other harmful gases to replace activated carbon and other synthetic polymers, used for this purpose.

References

1. A. O. Ashogbon and E. T. Akintayo, Starch/Staerke, 2014, 66, 41–57.
2. R. F. Tester, J. Karkalas and X. Qi, J. Cereal Sci., 2004, 39, 151–165.
3. S. Pérez and E. Bertoft, Starch/Staerke, 2010, 62, 389–420.
4. Y. Yi, Z. Yang and S. Zhang, Environ. Pollut., 2011, 159, 2575–2585.
5. L. Jarup, Br. Med. Bull., 2003, 68, 167–182.
6. M. M. N. Authman, J. Aquacult. Res. Dev., 2015, 06, 1000328.
7. E. Y. Ozmen, M. Sezgin, A. Yilmaz and M. Yilmaz, Bioresour. Technol., 2008, 99, 526–531.
8. Y. Yang, X. Wei, P. Sun and J. Wan, Molecules, 2010, 15, 2872–2885.
9. M. A. Barakat, Arabian J. Chem., 2011, 4, 361–377.
10. E. A. Deliyanni, G. Z. Kyzas, K. S. Triantafyllidis and K. A. Matis, Open Chem., 2015, 13, 699–708.
11. J.-Q. Jiang and S. M. Ashekuzzaman, Curr. Opin. Chem. Eng., 2012, 1, 191–199.
12. S. Babel and T. A. Kurniawan, J. Hazard. Mater., 2003, 97, 219–243.
13. D. K. Kweon, J. K. Choi, E. K. Kim and S. T. Lim, Carbohydr. Polym., 2001, 46, 171–177.
14. B. Xiang, W. Fan, X. Yi, Z. Wang, F. Gao, Y. Li and H. Gu, Carbohydr. Polym., 2016, 136, 30–37.
15. X. Ma, X. Liu, D. P. Anderson and P. R. Chang, Food Chem., 2015, 181, 133–139.
16. D. Soto, J. Urdaneta, K. Pernía, O. León, A. M. Bonilla and M. F. García, Starch/Staerke, 2016, 68, 37–46.
17. H. Peng, S. Zhong, Q. Lin, X. Yao, Z. Liang, M. Yang, G. Yin, Q. Liu and H. He, Carbohydr. Polym., 2016, 138, 210–214.
18. Z. S. Pour and M. Ghaemy, RSC Adv., 2015, 5, 64106–64118.
19. S. Xu, J. Wang, R. Wu, J. Wang and H. Li, Chem. Eng. J., 2006, 117, 161–167.
20. B. Kaur, F. Ariffin, R. Bhat and A. A. Karim, Food Hydrocolloids, 2012, 26, 398–404.
21. Q. Chen, H. Yu, L. Wang, Z. ul Abdin, Y. Chen, J. Wang, W. Zhou, X. Yang, R. U. Khan and H. Zhang, RSC Adv., 2015, 5, 67459–67474.
22. B. Tosh and C. R. Routray, Chem. Sci. Rev. Lett., 2014, 10, 74–92.
23. H. Kang, R. Liu and Y. Huang, Polymer, 2015, 70, 1–16.
24. D. Roy, M. Semsarilar, J. T. Guthrie and S. Perrier, Chem. Soc. Rev., 2009, 38, 2046–2064.
25. A. Bhattacharya, Prog. Polym. Sci., 2004, 29, 767–814.
26. V. D. Athawale and S. C. Rathi, J. Macromol. Sci., Polym. Rev., 1999, 39, 445–480.
27. S. B. Nair and A. N. Jyothi, J. Appl. Polym. Sci., 2014, 131, 39810.
28. V. Lele, Int. J. Adv. Res., 2015, 1, 107–110.
29. D. Apopei, M. Dinu and E. Dragan, Dig. J. Nanomater. Bios., 2012, 7, 707–716.
30. S. Mishra, A. Mukul, G. Sen and U. Jha, Int. J. Biol. Macromol., 2011, 48, 106–111.
31. L. Fakhru, A. Razi, I. Y. Qudsieh, W. M. Z. W. Yunus, M. B. Ahmad and M. Z. A. Rahman, J. Appl. Polym. Sci., 2001, 82, 1375–1381.
32. P. C. Parvathy and A. N. Jyothi, J. Appl. Polym. Sci., 2014, 131, 40368.
33. J. R. Witono, I. W. Noordergraaf, H. J. Heeres and L. P. Janssen, Carbohydr. Polym., 2012, 90, 1522–1529.
34. C. Banerjee, P. Gupta, S. Mishra, G. Sen, P. Shukla and R. Bandopadhyay, Int. J. Biol. Macromol., 2012, 51, 456–461.
35. A. D. Mohammed, D. A. Young and H. C. M. Vosloo, Starch/Staerke, 2014, 66, 393–399.
36. Q. Guo, Y. Wang, Y. Fan, X. Liu, S. Ren, Y. Wen and B. Shen, Carbohydr. Polym., 2015, 117, 247–254.
37. S. Djordjevic, L. Nikolic, S. Kovacevic, M. Miljkovic and D. Djordjevic, Period. Polytech., Chem. Eng., 2013, 57, 55–61.
38. S. Djordjevic, L. Nikolic, S. Kovacevic, M. Miljkovic and D. Djordjevic, Chem. Ind. Chem. Eng. Q., 2013, 19, 493–503.
39. M. R. Vakili and N. Rahneshin, Carbohydr. Polym., 2013, 98, 1624–1630.
40. J. P. Wang, S. J. Yuan, Y. Wang and H. Q. Yu, Water Res., 2013, 47, 2643–2648.
41. M. Teli and A. Mallick, J. Eng. Technol. Res., 2015, 2, 442–448.
42. H. Song, Carbohydr. Polym., 2010, 82, 768–771.
43. M. Liu, R. Cheng, J. Wu and C. Ma, J. Polym. Sci., Part A: Polym. Chem., 1993, 31, 3181–3186.
44. M. Worzakowska and M. Grochowicz, Carbohydr. Polym., 2015, 130, 344–352.
45. G. F. Fanta, R. Burr, W. Doane and C. Russell, J. Appl. Polym. Sci., 1979, 23, 229–240.
46. N. Sheikh, A. Akhavan and E. Ataeivarjovi, Radiat. Phys. Chem., 2013, 85, 189–192.
47. S. Zhang, W. Wang, H. Wang, W. Qi, L. Yue and Q. Ye, Carbohydr. Polym., 2014, 101, 798–803.
48. H. L. A. El-Mohdy, E. A. Hegazy, E. M. El-Nesr and M. A. El-Wahab, Arabian J. Chem., 2012 DOI:10.1016/j.arabjc.2012.04.022.
49. S. Wang, Q. Wang, X. Fan, J. Xu, Y. Zhang, J. Yuan, H. Jin and A. Cavaco-Paulo, Carbohydr. Polym., 2016, 136, 1010–1016.
50. R. L. Shogren, J. L. Willett and A. Biswas, Carbohydr. Polym., 2009, 75, 189–191.
51. S. Lv, R. Gong and Y. Ma, Polym. Adv. Technol., 2012, 23, 1343–1349.
52. S. Lv, T. Sun, Q. Zhou, J. Liu and H. Ding, Carbohydr. Polym., 2014, 103, 285–293.
53. M. Szwarc, J. Polym. Sci., Part A: Polym. Chem., 1998, 36, IX–XV.
54. L. Wang, J. Shen, Y. Men, Y. Wu, Q. Peng, X. Wang, R. Yang, K. Mahmood and Z. Liu, Polym. Chem., 2015, 6, 3480–3488.
55. A. Bansal, A. Kumar, P. P. Latha, S. S. Ray and A. K. Chatterjee, Carbohydr. Polym., 2015, 130, 290–298.
56. P. Liu and Z. Su, Carbohydr. Polym., 2005, 62, 159–163.
57. L. Wang, Y. Wu, Y. Men, J. Shen and Z. Liu, RSC Adv., 2015, 5, 70758–70765.
58. P. Tomasik and C. H. Schilling, Adv. carbohydr. chem. biochem., Academic Press, 2004, 59, pp. 175–403.
59. M. Tahan and A. Zilkha, J. Polym. Sci., Part A: Polym. Chem., 1969, 7, 1825–1837.
60. D. R. Houzé, P. Degée, R. Gouttebaron, M. Hecq, R. Narayan and P. Dubois, Polym. Int., 2004, 53, 656–663.
61. G. Ezra and A. Zilkha, J. Polym. Sci., Part A: Polym. Chem., 1970, 8, 1343–1356.
62. E. Cohen and A. Zilkha, Isr. J. Chem., 1973, 11, 45–51.
63. E. Cohen and A. Zilkha, J. Polym. Sci., Part A: Polym. Chem., 1969, 7, 1881–1892.
64. A. N. Jyothi, S. N. Moorthy and K. N. Rajasekharan, Starch/Staerke, 2006, 58, 292–299.
65. L. Guo, G. Li, J. Liu, Y. Meng and Y. Tang, Carbohydr. Polym., 2013, 93, 374–379.
66. J. Hu, T. Tian and Z. Xiao, Polym. Adv. Technol., 2015, 26, 1259–1266.
67. A. V. Singh and L. K. Nath, Int. J. Biol. Macromol., 2012, 50, 14–18.
68. H.-S. Kim, D. K. Hwang, B. Y. Kim and M. Y. Baik, Food Chem., 2012, 130, 977–980.
69. J. S. Hong, S. V. Gomand and J. A. Delcour, Carbohydr. Polym., 2015, 124, 302–310.
70. F. M. Carbinatto, A. D. de Castro, B. S. Cury, A. Magalhaes and R. C. Evangelista, Int. J. Pharm., 2012, 423, 281–288.
71. R. Wongsagonsup, T. Pujchakarn, S. Jitrakbumrung, W. Chaiwat, A. Fuongfuchat, S. Varavinit, S. Dangtip and M. Suphantharika, Carbohydr. Polym., 2014, 101, 656–665.
72. N. Reddy and Y. Yang, Food Chem., 2010, 118, 702–711.
73. J. Fang, Carbohydr. Polym., 2004, 55, 283–289.
74. H. Chi, K. Xu, X. Wu, Q. Chen, D. Xue, C. Song, W. Zhang and P. Wang, Food Chem., 2008, 106, 923–928.
75. J. Q. Mei, D. N. Zhou, Z. Y. Jin, X. M. Xu and H. Q. Chen, Food Chem., 2015, 187, 378–384.
76. A. Zarski, S. Ptak, P. Siemion and J. Kapusniak, Carbohydr. Polym., 2016, 137, 657–663.
77. Y. F. Zuo, J. Gu, Z. Qiao, H. Tan, J. Cao and Y. Zhang, Int. J. Biol. Macromol., 2015, 72, 391–402.
78. J. Xu, C. W. Zhou, R. Z. Wang, L. Yang, S. S. Du, F. P. Wang, H. Ruan and G. Q. He, Carbohydr. Polym., 2012, 87, 2137–2144.
79. H. Granö, J. Y. Kauhaluoma, T. Suortti, J. Käki and K. Nurmi, Carbohydr. Polym., 2000, 41, 277–283.
80. A. A. M. L. Huijbrechts, J. Huang, H. A. Schols, B. V. Lagen, G. M. Visser, C. G. Boeriu and E. J. R. Sudhölter, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 2734–2744.
81. V. Singh, A. K. Sharma, D. N. Tripathi and R. Sanghi, J. Hazard. Mater., 2009, 161, 955–966.
82. Y. Shi, B. Ju and S. Zhang, Carbohydr. Polym., 2012, 88, 132–138.
83. O. C. Wokadala, N. M. Emmambux and S. S. Ray, Carbohydr. Polym., 2014, 112, 216–224.
84. M. A. Misman, A. R. Azura and Z. A. A. Hamid, Ind. Crops Prod., 2015, 65, 397–405.
85. N. Teramoto, T. Motoyama, R. Yosomiya and M. Shibata, Eur. Polym. J., 2003, 39, 255–261.
86. O. B. Wurzburg, Modified starches-properties and uses, CRC Press Inc., 1986.
87. S. Pal, D. Mal and R. P. Singh, Carbohydr. Polym., 2005, 59, 417–423.
88. R. Kavaliauskaite, R. Klimaviciute and A. Zemaitaitis, Carbohydr. Polym., 2008, 73, 665–675.
89. Y. Jiang, B. Ju, S. Zhang and J. Yang, Carbohydr. Polym., 2010, 80, 467–473.
90. T. Heinze, V. Haack and S. Rensing, Starch/Staerke, 2004, 56, 288–296.
91. Y. Wei, F. Cheng and H. Zheng, Carbohydr. Polym., 2008, 74, 673–679.
92. M. I. Khalil and A. A. Aly, Starch/Staerke, 2002, 54, 132–139.
93. W. Yanli, G. Wenyuan and L. Xia, Carbohydr. Res., 2009, 344, 1764–1769.
94. L. F. Wang, S. Y. Pan, H. Hu, W. H. Miao and X. Y. Xu, Carbohydr. Polym., 2010, 80, 174–179.
95. Q. Lin, S. Qian, C. Li, H. Pan, Z. Wu and G. Liu, Carbohydr. Polym., 2012, 90, 275–283.
96. K. N. Waliszewski, M. A. Aparicio, L. S. A. Bello and J. A. Monroy, Carbohydr. Polym., 2003, 52, 237–242.
97. Y. Zhou, S. Meng, D. Chen, X. Zhu and H. Yuan, Carbohydr. Polym., 2014, 103, 81–86.
98. A. Karim, E. Sufha and I. Zaidul, J. Agric. Food Chem., 2008, 56, 10901–10907.
99. C. S. Raina, S. Singh, A. S. Bawa and D. C. Saxena, Eur. Food Res. Technol., 2006, 223, 561–570.
100. S. Wattanachant, S. K. S. Muhammad, D. M. Hashim and R. A. Rahman, Songklanakarin J. Sci. Technol., 2002, 24, 431–438.
101. H. X. Xiao, Q. L. Lin, G. Q. Liu and F. X. Yu, Molecules, 2012, 17, 10946–10957.
102. S. Sukhija, S. Singh and C. S. Riar, Food Hydrocolloids, 2016, 55, 56–64.
103. J. Zhao, Z. Chen, Z. Jin, P. Buwalda, H. Gruppen and H. A. Schols, J. Agric. Food Chem., 2015, 63, 4646–4654.
104. S. Mehboob, T. M. Ali, F. Alam and A. Hasnain, LWT--Food Sci. Technol., 2015, 64, 459–467.
105. S. J. Lee, J. Y. Hong, E. J. Lee, H. J. Chung and S. T. Lim, Carbohydr. Polym., 2015, 122, 77–83.
106. J. Liu, B. Wang, L. Lin, J. Zhang, W. Liu, J. Xie and Y. Ding, Food Hydrocolloids, 2014, 36, 45–52.
107. T. Woggum and T. Wittaya, Food and Applied Bioscience Journal, 2015, 3, 247–266.
108. O. S. Kittipongpatana and N. Kittipongpatana, Food Chem., 2013, 141, 1438–1444.
109. P. B. Z. Flores, S. B. Baños, R. S. Delgado and L. A. B. Pérez, J. Appl. Polym. Sci., 2009, 112, 822–829.
110. A. B. Das, G. Singh, S. Singh and C. S. Riar, Carbohydr. Polym., 2010, 80, 725–732.
111. A. Dong, J. Xie, W. Wang, L. Yu, Q. Liu and Y. Yin, J. Hazard. Mater., 2010, 181, 448–454.
112. D. K. Kweon, J. K. Choi, E. K. Kim and S. T. Lim, Carbohydr. Polym., 2001, 46, 171–177.
113. B. Kim and S. T. Lim, Carbohydr. Polym., 1999, 39, 217–223.
114. G. Güçlü, E. Al, S. Emik, T. B. İyim, S. Özgümüş and M. Özyürek, Polym. Bull., 2009, 65, 333–346.
115. S. Keleş and G. Güçlü, Polym.-Plast. Technol. Eng., 2006, 45, 365–371.
116. S. M. Xu, S. Feng, G. Peng, J. D. Wang and A. Yushan, Carbohydr. Polym., 2005, 60, 301–305.
117. S. M. Xu, S. F. Zhang, R. W. Lu, J. Z. Yang and C. X. Cui, J. Appl. Polym. Sci., 2003, 89, 263–267.
118. B. Sancey, G. Trunfio, J. Charles, J. F. Minary, S. Gavoille, P. M. Badot and G. Crini, J. Environ. Manage., 2011, 92, 765–772.
119. L. Guo, S. F. Zhang, B. Z. Ju, J. Z. Yang and X. Quan, J. Polym. Res., 2006, 13, 213–217.
120. W. Ding, P. Zhao and R. Li, Carbohydr. Polym., 2011, 83, 802–807.
121. J.-T. Liu, K.-Y. Zhang, P. Wang, W. Ding, P. Zhao, Z.-P. Dong and R. Li, Sep. Sci. Technol., 2013, 48, 766–774.
122. Y. Zheng, S. Hua and A. Wang, Desalination, 2010, 263, 170–175.
123. Y. J. Li, B. Xiang and Y. M. Ni, J. Appl. Polym. Sci., 2004, 92, 3881–3885.
124. M. I. Khalil and M. G. A. Halim, Starch/Staerke, 2001, 53, 35–41.
125. H. Kolya and T. Tripathy, Int. J. Biol. Macromol., 2013, 62, 557–564.
126. X. Cheng, R. Cheng, S. Ou and Y. Li, Carbohydr. Polym., 2013, 96, 320–325.
127. G. Xie, X. Shang, R. Liu, J. Hu and S. Liao, Carbohydr. Polym., 2011, 84, 430–438.
128. Q. Chang, X. Hao and L. Duan, J. Hazard. Mater., 2008, 159, 548–553.
129. M. Irani, H. Ismail, Z. Ahmad and M. Fan, J. Environ. Sci., 2015, 27, 9–20.
130. K. N. Awokoya and B. A. Moronkola, Int. J. Eng. Sci., 2012, 1, 18–24.
131. G. Güçlü, S. Keleş and K. Güçlü, Polym.-Plast. Technol. Eng., 2006, 45, 55–59.
132. G. Güçlü, K. Güçlü and S. Keleş, J. Appl. Polym. Sci., 2007, 106, 1800–1805.
133. E. S. A. Halim and S. S. Al-Deyab, React. Funct. Polym., 2014, 75, 1–8.
134. S. M. Xu, S. Feng, F. Yue and J. D. Wang, J. Appl. Polym. Sci., 2004, 92, 728–732.
135. H. Kolya, A. Roy and T. Tripathy, Int. J. Biol. Macromol., 2015, 72, 560–568.
136. X. Hao, Q. Chang, L. Duan and Y. Zhang, Starch/Staerke, 2007, 59, 251–257.
137. E. S. Dragan and D. F. Apopei Loghin, Chem. Eng. J., 2013, 234, 211–222.
138. G. Crini, Bioresour. Technol., 2006, 97, 1061–1085.
139. M. S. Zafar, M. Tausif, M. Mohsin, S. W. Ahmad and M. Zia-ul-Haq, Water, Air, Soil Pollut., 2015, 226, 244.
140. T. Cai, Z. Yang, H. Li, H. Yang, A. Li and R. Cheng, Cellulose, 2013, 20, 2605–2614.
141. G. Zhang, L. Yi, H. Deng and P. Sun, J. Environ. Sci., 2014, 26, 1203–1211.
142. V. Panic, S. Seslija, A. Nesic and S. Velickovic, Hem. Ind., 2013, 67, 881–900.
143. M. Kıranşan, R. D. C. Soltani, A. Hassani, S. Karaca and A. Khataee, J. Taiwan Inst. Chem. Eng., 2014, 45, 2565–2577.
144. V. K. Gupta and Suhas, J. Environ. Manage., 2009, 90, 2313–2342.
145. R. Cheng, B. Xiang, Y. Li and M. Zhang, J. Hazard. Mater., 2011, 188, 254–260.
146. R. Cheng, B. Xiang and Y. Li, J. Appl. Polym. Sci., 2012, 123, 2439–2444.
147. Z. Wang, B. Xiang, R. Cheng and Y. Li, J. Hazard. Mater., 2010, 183, 224–232.
148. R. Cheng, S. Ou, M. Li, Y. Li and B. Xiang, J. Hazard. Mater., 2009, 172, 1665–1670.
149. X. Shimei, W. Jingli, W. Ronglan and W. Jide, Carbohydr. Polym., 2006, 66, 55–59.
150. R. F. Gomes, A. C. de Azevedo, A. G. Pereira, E. C. Muniz, A. R. Fajardo and F. H. Rodrigues, J. Colloid Interface Sci., 2015, 454, 200–209.
151. R. Chen, Y. Zhang, L. Shen, X. Wang, J. Chen, A. Ma and W. Jiang, Chem. Eng. J., 2015, 268, 348–355.
152. A. A. Hebeish and A. A. Aly, Int. J. Org. Chem., 2014, 04, 208–217.
153. H. Kolya and T. Tripathy, Eur. Polym. J., 2013, 49, 4265–4275.
154. E. S. A. Halim, React. Funct. Polym., 2013, 73, 1531–1536.
155. G. Güçlü and S. Keleş, J. Appl. Polym. Sci., 2007, 106, 2422–2426.
156. E. Al, G. Güçlü, T. B. İyim, S. Emik and S. Özgümüş, J. Appl. Polym. Sci., 2008, 109, 16–22.
157. R. Klimaviciute, A. Riauka and A. Zemaitaitis, J. Polym. Res., 2006, 14, 67–73.

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