Preparation and Characterization of Calcium Cross-Linked Starch Monolithic Cryogels and Their Application as Cost-Effective Green Filters

Abstract

Monolithic cryogels from starch were successfully synthesized and applied as alternative biodegradable filters for the first time. Rice flour was cross-linked with Ca2+ from limewater during gelatinization before being frozen and then thawed for three cycles. The resultant material was then soaked in ethanol for 3 h before incubation at 80 °C for 1 h, yielding monolithic material with interconnected pores in sizes of 51 ± 18 to 52 ± 15 µm without any need of freeze-drying. The cryogels possessed macroporous structure with specific surface areas from 1.1 to 4.3 m2g−1, they could adsorb water from 599 ± 27 to 635 ± 59% of their dry weight with low swelling ratios of 6.0 ± 0.3 to 6.4 ± 0.6 gwater/gcryogel, and could be applied as biofilters to remove suspended particles and reduce the light absorption of water sample from 25 ± 3 to 96 ± 5%. The prepared biofilters can be re-used up to three times, although they cost only USD 0.0004/piece. Complete weight loss resulted from burial in soil for 30 days, indicating environmentally friendly biodegradation and potential for environmental applications.

Continuous Phosphate Removal and Recovery Using a Calcium Silicate Hydrate Composite Monolithic Cryogel Column

“…The absorption band at 1643 cm −1 was attributed to H–O–H bending in water molecules in CSH which may overlap with C–O bending in amylopectin in starch [ 10 , 49 , 50 ]. The remaining intense band at ~998 cm −1 before and after phosphate adsorption was attributed to O–Si–O stretching vibrations from CSH molecules [ 10 , 34 ], which overlap with C–O–H bending in amylopectin [ 49 , 50 , 51 ]. The highly intense band of Cry–CSH after adsorption at ~1077 cm −1 can be attributed to the C–O, C–C, and O–H bond stretching bands of starch [ 49 , 50 , 51 ] that may overlap with the vibration bands of H 2 PO 4 – and HPO 4 2– commonly appearing at 1038 cm −1 [ 33 ] or 1046 cm −1 [ 35 ].…”

Preparation and Characterization of Biodegradable Sponge-like Cryogel Particles of Chitosan via the Inverse Leidenfrost (iLF) Effect

“…The greater cross-linking degree increased the cryogel’s network density and caused a slow relaxation of the network chains. This circumstance has caused a more difficult network expansion, thus decreasing the SD . When the amount of acetic acid was 0.01 mol, the SD of cryogels was the highest because the cross-linking reaction between chitosan and glutaraldehyde had not fully occurred due to the incomplete dissolution of the chitosan chain polymer.…”

“…This circumstance has caused a more difficult network expansion, thus decreasing the SD. 32 When the amount of acetic acid was 0.01 mol, the SD of cryogels was the highest because the cross-linking reaction between chitosan and glutaraldehyde had not fully occurred due to the incomplete dissolution of the chitosan chain polymer. The SD remained relatively unchanged for 0.03 and 0.04 mol of acetic acid, marking that the SD had reached equilibrium and the cross-linking reaction had fully completed.…”

Recovering Phosphate from Complex Wastewater Using Macroporous Cryogel Composited Calcium Silicate Hydrate Nanoparticles

“…The characterization of the CSH column prephosphate adsorption using the FTIR method revealed bands at 998 and 663 cm −1 ( Figure 3 a), indicating O–Si–O stretching and Si–O–Si bending vibrations from the unique silicon (Si)–oxygen (O) tetrahedral structure of CSH molecules [ 23 , 24 , 40 , 41 ] and confirming the immobilization of CSH within the cryogel network, as shown in the FESEM images ( Figure 1 b). The C–O–H bending of amylopectin from starch cryogel was observed at 1013 cm −1 in the CSH column before phosphate adsorption [ 31 ], whereas it may have overlapped with H 2 PO 4 − and HPO 4 2− V3 band vibration and presented as more intense peaks at 1016 cm −1 [ 20 , 41 ] postphosphate adsorption ( Figure 3 b). Moreover, a new intense peak of P–O vibration was observed at 562 cm −1 , indicating phosphate adsorption on the material.…”

“…The CSH nanoparticles (pore diameter: ~1.05 nm [ 23 ]) were dried in an oven before use. They were homogeneously mixed with gelatinized starch (4.5-g CSH:60-g gelatinized starch) before being filled in a plastic syringe (volume: 50 mL; diameter: 3 cm; length: 10 cm) to prepare the monolithic cryogel composited CSH nanoparticle column (CSH column) using the freeze–thaw method [ 23 , 24 , 31 ]. The obtained CSH column was removed from the container before it was cut into desired lengths (2.5, 5.0, and 7.5 cm) and put back into a plastic syringe preinstallation in a continuous-flow system.…”