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 g_water/g_cryogel, 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.

Keywords: biodegradable cryogel; biodegradable filter; limewater; macropore; starch.

Figure 1

(a) The cryogels obtained with various amounts of red lime (RL), white lime (WL) and calcium hydroxide standard solution (Ca(OH)₂) prepared using the limewater on the day of its preparation with one freeze–thaw-cycle, and (b) their FE-SEM images.

Figure 2

FE-SEM images of the cryogels prepared using various amounts of Ca(OH)₂ in limewater prepared at least three days earlier and with three freeze thaw cycles.

Figure 3

FE-SEM images and average pore sizes of the cryogels prepared by using (a–c,f–i) 0.32% RL with 1–7 freeze–thaw cycles, respectively (d) 0.16% Ca(OH)₂, and (e) ultrapure water with three freeze–thaw cycles under the optimal conditions.

Figure 4

Proposed mechanism of synthesis of the cryogels.

Figure 5

Nitrogen adsorption-desorption isotherms of the cryogels prepared from (a) 0.16% Ca(OH)₂, and (b) 0.32% RL; (c) XRD and, (d) TGA patterns of the synthesized cryogels.

Figure 6

FTIR spectra of the cryogels.

Figure 7

Swelling behavior of the cryogels prepared using 0.32% RL and 0.16% Ca(OH)₂ (n = 3) (a) swelling ratio, (b) effects of pH, (c) water uptake capacity, and (d) water retention.

Figure 8

(a) Real water samples before and after filtering through the cryogels; (b) the cryogels after used to filter real samples; (c) reusability of the filters.