Due to economic development, the application of natural and synthetic dyes has expanded in food and non-food industrial fields. However, approximately 10% of the dyes used are wasted and discharged into aquatic systems, leading to serious threats to the environment and public health. Thus, the removal of dyes from wastewater is of great importance for environmental protection. A decolorization process is widely used in either wastewater treatment or the manufacture of industrial products. Compared with other decolorization technologies, such as biological treatments, advanced oxidation processes, and photocatalytic degradation, adsorption is considered a safe technology without causing secondary pollution. The preparation of safer, more stable, and efficient absorbents is essential to improve the application of adsorption technology. In recent years, starch-based adsorbents, which are safer for dye removal, have been a focus of attention in the development of biodegradable and safe adsorbents.
- Introduction to Porous Starch (PS)
- Porous Starch Preparation
- Porous Starch Structure and Properties
- Mechanism of the Formation of Pores in Starch
- Application of Porous Starch
Schneider P, Smith J prepared cross-linked porous starch (CPS) by hydrolyzing cross-linked starch (native starch cross-linked with epichlorohydrin) with α-amylase and applied this CPS, as a biodegradable and safe adsorbent, for the removal of methylene blue (MB) from aqueous solution. CPS showed higher adsorption capacity than native starch. In addition, investigations showed that the adsorption of MB on CPS was endothermic and spontaneous in nature. The equilibrium adsorption data were well described by the Langmuir isotherm model with a maximum adsorption capacity of 9.46 mg/g under the conditions of a MB initial concentration of 10 mg/L at 20 °C. Chang PR, Qian D compared MB adsorption capacity between PS and SA-modified PSs (SAPSs) obtained by modification of PSs with succinic anhydride (SA) in a solvent-/catalystfree medium. The results showed that the capacity of MB adsorption by SAPSs increased gradually with increasing degree of substitution. In a system containing 10 g/L of SAPS and 0.0854 mmol/L (31.93 mg/L) MB dye, SAPs showed better adsorption than PS without substitution, 2.6 mg/g and 1.6 mg/g for SAPs with a degree of substitution of 1.9 and PS, respectively.
In order to remove hazardous gardenia yellow (GY), a natural colorant which is genotoxic, Bao L, Zhu X, Dai H modified potato starch with mercaptosuccinic acid (MSA) during its gelatinization by breaking the intrinsic hydrogen bonds within the macromolecular chains of starch. Porous starch xerogels (PSX) were obtained after freeze-drying, by inserting MSA molecules into the starch chains. The obtained PSX/MSA exhibited a porous structure due to the existence of MSA molecules within the macromolecular chains of starch. The results showed that the number of macropores within the PSX/MSA samples increased when the MSA content in the composites increased and improved the adsorption of GY. Adsorption of GY by PSX/MSA adsorbents was markedly higher, with a capacity of 72 mg/g, compared with natural potato starch and starch xerogels alone, with a capacity of less than 10 mg/g. These results were due to the different structures of these adsorbents. In the case of potato starch, the movement of GY molecules was categorized into surface diffusion over the solid surface of potato starch; however, in the case of PSX/ MSA adsorbents, both pore and surface diffusion occurred due to the presence of macropores. According to Schneider P, Smith J, as the activation energy for diffusion was lower than the energy of adsorption, the contribution of surface diffusion was negligible when pore diffusion occurred. Thus, pore diffusion made a significant contribution to the adsorption of GY by these PSX/MSA adsorbents.
Heavy Metal Ion Removal
Heavy metal ions, such as Cu2+, Cd2+, and Pb2+, pose a threat to public health and environmental safety. Recently, various adsorbents, including oxidized carbon nanotubes, graphene oxide, clay, and biomass sources, have been used to remove heavy metals from aqueous solutions. Considering their low cost, availability, and biodegradability, biomass-based adsorbents are preferred. Natural polysaccharidebased adsorbents have been shown to efficiently remove heavy metal ions. However, adsorbents prepared from polysaccharides are usually soluble or in powder form, leading to the difficulty of separation after the adsorption process. PS, which has the advantages of being biodegradable with easy and economical solid–liquid separation, is now attracting the interest of researchers.
By reacting PS with carbon disulfide, and PS with citric acid, Ma et al. (2015) prepared porous starch xanthate (PSX) and porous starch citrate (PSC), respectively. The attached xanthate and carboxylate groups formed chelation and electrostatic interactions, respectively, with heavy metal ions during the adsorption process. The adsorption capacity of PSX and PSC for lead(II) ions was evaluated in a system containing 2 g/L of the adsorbents and PbNO3 solution (0.5 g/L Pb2+). The adsorption capacity was found to be highly dependent on the carbon disulfide/starch and citric acid/starch mole ratios used during preparation. The maximum adsorption capacity for PSX and PSC was 109.1 mg/g and 57.6 mg/g, respectively, and showed promising efficiency for the removal of heavy metals from contaminated liquids. Naushad M, Ahamad T prepared a starch-based nanocomposite (starch/SnO2) as an effective adsorbent to remove Hg2+ from aqueous medium. The adsorption performance was demonstrated by an increase in temperature, indicating an endothermic adsorption. At room temperature (25 °C), the maximum adsorption capacity was 192 mg/g.