Depending on the source from which starch is obtained, the proportion of amylose and amylopectin varies, and so does its average molecular weight
[99][178]. Some of the most common sources include maize, potato, wheat and rice. Nevertheless, there are promising unconventional sources under study, such as fruits and vegetables waste. Kringel et al. conducted studies on amylose and amylopectin content of waste from fruits and vegetables with the aim of analysing its suitability as valorised residues from natural sources. The reported starch content of outer pericarp and core tissues of kiwi, mango kernels, pineapple stems, apple pulp, banana peels and avocado seeds was in a 20–50% range
[100][179].
Regarding water treatment, native starch has been reported to have a poor adsorption capacity
[101][180], which can mainly be attributed to its low surface area, limited thermal stability and absence of highly-absorptive functional groups, such as carboxyl, xanthate, acrylate, acetyl, hydroxypropyl, amine or amide
[102][103][181,182]. Analogously to cellulose, alginate and chitosan, there are reports describing starch’s chemical modifications as well as the preparation of starch composites.
For instance, Guo et al. compared the efficiency for MB removal attained by native starch and a modified porous derivative, prepared by crosslinking native starch with epichlorohydrin and further hydrolysis with α-amylase, obtaining a maximum adsorption capacity value for MB almost three times higher than native starch
[104][183]. In accordance with the results reported by Guo et al., Alvarado et al. also observed an enhancement of the adsorption capacity for the MB of starch after being modified by esterification with malonic, glutaric and valeric acids
[105][184].
Soto et al. also compared modified and native starch. In this case, maleic acid and itaconic acids were used for esterification of starch, achieving maximum adsorption capacity values for Pb (II) and Zn (II) of 25.2 and 7.9 mg g
−1, respectively for itaconate starch, while 5.2 and 3.2 mg g
−1 were the corresponding values for native starch
[106][185]. Soto et al. also reported oxidized starch adsorbents that achieved higher heavy metal removal capacity values compared to native starch; for Cd (II): 11.1 and 8.0 mg g
−1; for Ni (II): 22.6 and 8.3 mg g
−1; for Pb (II): 18.0 and 5.2 mg g
−1; and for Zn (II): 10.3 and 3.2 mg g
−1, respectively
[107][186].
Sancey et al. reported the crosslinking of starch with 1,4-butanediol diglycidylether in presence of ammonium hydroxide and 2,3-epoxy-propyltrimethylammonium chloride, with further carboxymethylation. Herein, metal adsorption studies were conducted, achieving almost complete removal of Cu (II) and Fe (III)
[108][187]. Copper removal was also assayed by Zheng et al. using starch hydrogels prepared with poly(acrylic acid), achieving an uptake of 179.9 mg g
−1 Cu(II) in aqueous medium
[109][188].
Chaudhari et al. prepared a starch xanthate that achieved almost complete removal of Hg (II), Cu (II), Cd (II) and Ni (II), and attributed the heavy metals uptake to the complexation of metals with xanthate groups
[110][189].
As far as starch composites are concerned, composites with layer double hydroxide for methyl orange and fluoride removal from wastewater were reported. These materials were prepared by co-precipitation and different metals were used for the hydroxide layers, such as Mg (II), Al (III), Zn (II), Fe (III) and Ni (II). In particular, Zubair et al. reported a Starch-Ni/Fe-layered double hydroxide composite that achieved a maximum methyl orange (MO) adsorption capacity value of 387.6 mg g
−1 [111][190]. However, Tao et al. achieved a higher maximum adsorption capacity value towards MO of 1555.0 mg g
−1 [112][191] for the Zn/Mg/Al- Layered double hydroxide starch. On the other hand, a Mg/Al layered double hydroxides reported by Liu et al. achieved almost complete removal of fluoride
[113][192].
Besides, iron nanoparticles are well-known to contribute to arsenic removal due to electrostatic attraction, ion exchange, and surface complexation
[114][193]. It is therefore interesting to analyze iron nanoparticles’ effect when used in materials prepared with starch, such as composite materials with magnetite, maghemite and ferromanganese binary oxide. Next, some of the reported materials are described, in which starch acted only as a support or stabilizer of the mentioned inorganic particles.
Adsorption studies towards As (III) on iron-starch materials were carried out in aqueous solutions, achieving a maximum adsorption capacity value of 161.3 mg g
−1 with the material reported by Xu et al.
[115][194], while Siddiqui et al., obtained a maximum capacity value of 8.9 mg g
−1 for a functionalized maghemite
[116][195] and Robinson et al., 55.9 mg g
−1 for a starch/maghemite nano-adsorbent
[117][196].
On the other hand, as in the case of the other polysaccharides mentioned in previous sections, the preparation of ion exchangers from starch is a widely used strategy for ionic pollutants removal. For instance, Ma et al.
[118][197] chemically modified starch attaching either xanthate or citrate groups to their structure (
Figure 7I,II). In this work, adsorption assays towards Pb (II) were carried out, and the maximum adsorption capacity values were 109.1 and 57.6 mg g
−1 for starch xanthate and starch citrate, respectively.
Figure 7.
Chemical structures of cation exchangers derived from starch.
Another strategy for introducing negative charges in the starch structure is the oxidation of C-6 to a carboxyl group (
Figure 7III). In this context, Liu et al.
[119][198] modified starch nanoparticles through oxidization, using sodium hypochlorite. The new materials were preliminarily explored as adsorbents of heavy metals, using Pb (II) and Cu (II) as adsorbate models. Compared to the unmodified nanoparticles, the modified ones had remarkably higher adsorption capacity values (94.5 and 81.0 mg g
−1 for Pb (II) and Cu (II), respectively).
Chen et al.
[120][199] also used starch nanoparticles, but in this case the incorporation of negative charges in their surface was achieved by esterification with succinyl anhydride (
Figure 7IV). The maximum degree of substitution was 0.1, which resulted in an increase on the surface Z-potential. The modified nanoparticles were assayed for the adsorption of Cu (II) and MB, showing that the adsorption process was greatly influenced by the pH value, as expected in presence of carboxyl groups. At low pH values, carboxyl groups are highly protonated, and the adsorption could be neglected (less than 3.0 mg g
−1). At pH values above 4.0, the degree of protonation decreases and the surface charge of the adsorbents becomes negative, exerting electrostatic attraction to the adsorbate
[34][38][34,38], which enhances the adsorption capacity of Cu (II) and MB (8.4 and 24.4 mg g
−1, respectively).
Another example of modifications to obtain negatively charged starch was performed by Hashem et al.
[121][200], who reported starch hydrogels prepared via graft polymerization of acrylonitrile, followed by nitrile groups hydrolysis to generate carboxylic acids and amides (
Figure 7V). In this work, adsorption of Hg (II) on the starch hydrogels were carried out and a maximum adsorption capacity value of 1250 mg g
−1 was reported by the authors. Subsequently, the same author reported adsorption studies against Pb (II) carried out on a similar material. In this last case, the value of the maximum adsorption capacity value reported was 264.4 mg g
−1 [122][201].
Haroon et al.
[123][202] also obtained cation exchangers by grafting carboxymethylated starch with
N-vinylpyrrolidone (
Figure 7VI). The new material was assayed as adsorbent towards rhodamine 6G from wastewater with a maximum adsorption capacity value of 363.9 mg g
−1.
There are also reports of materials prepared using acrylic acid as the grafting monomer. For instance, Bahadoran et al.
[124][82] reported a starch-g-poly(acrylic acid) hydrogel, achieving 736.0 mg g
−1 of Cu (II) removal. Additionally, cellulose nanofibers were incorporated into the hydrogel, which enhanced its adsorption capacity, obtaining a value of 957.0 mg g
−1 of Cu (II).
Ma et al., also prepared a composite material from starch, obtaining a starch-graft-poly(acrylic acid)/organo-modified zeolite, which was assayed for Cr (III) adsorption. The adsorbent’s removal of the mentioned metal was 651.4 mg g
−1 [125][203].
Moreover, Chen et al.
[126][204] prepared a starch-based high-performance adsorptive hydrogel by grafting polyacrylic acid onto starch, and then crosslinking it with
N,
N’-methylene-bisacrylamide (
Figure 7VII). This adsorbent was employed to remove the organic cationic dye MB from effluents in which its concentration was high, yielding a maximum adsorption capacity value of 2967.7 mg g
−1.
Additionally, Shoaib et al. prepared a composite material using starch, acrylic acid, and activated carbon from the red alga
Pterocladia capillacea with
N,
N′-methylenebisacrylamide as a crosslinker and ammonium persulphate as an initiator. The adsorbent was assayed for MB dye removal, achieving 1428.6 mg g
−1 as the maximum adsorption capacity value
[127][205].
On the other hand, anion exchangers have been obtained by incorporating positive charges into the starch structure by introducing amino groups.