Modular Chitosan-Based Adsorbents for Tunable Uptake of Sulfate from Water

The context of this study responds to the need for sorbent technology development to address the controlled removal of inorganic sulfate (SO42−) from saline water and the promising potential of chitosan as a carrier system for organosulfates in pharmaceutical and nutraceutical applications. This study aims to address the controlled removal of sulfate using chitosan as a sustainable biopolymer platform, where a modular synthetic approach was used for chitosan bead preparation that displays tunable sulfate uptake. The beads were prepared via phase-inversion synthesis, followed by cross-linking with glutaraldehyde, and impregnation of Ca2+ ions. The sulfate adsorption properties of the beads were studied at pH 5 and variable sulfate levels (50–1000 ppm), where beads with low cross-linking showed moderate sulfate uptake (35 mg/g), while cross-linked beads imbibed with Ca2+ had greater sulfate adsorption (140 mg/g). Bead stability, adsorption properties, and the point-of-zero charge (PZC) from 6.5 to 6.8 were found to depend on the cross-linking ratio and the presence of Ca2+. The beads were regenerated over multiple adsorption-desorption cycles to demonstrate the favorable uptake properties and bead stability. This study contributes to the development of chitosan-based adsorbent technology via a modular materials design strategy for the controlled removal of sulfate. The results of this study are relevant to diverse pharmaceutical and nutraceutical applications that range from the controlled removal of dextran sulfate from water to the controlled release of chondroitin sulfate.

: Adsorption of inorganic sulfate on chitosan bead systems with and without cross-linking and cation imbibing.

Bead System Qe (mg/g) No Modification
<10 CL-ratio 1:2 w/o imbibing <10 CL-ratio 1:32 w/ calcium imbibing ≈ 10 CL-ratio 1:4 w/ Fe(III) imbibing 55 CL-ratio 1:4 w/ Ca imbibing 42 As expected, cross-linking alone does not lead to significant increase in sulfate uptake and remains below the detection limit of the chosen analytical method. However, a bead system with very little cross-linking but calcium imbibing indicated slight sulfate adsorption around the detection limit. With increased cross-linking (CL-ratio 1:4) and imbibing two different metals, the uptake performance in comparison to the purely cross-linked material (CL-ratio 1:2) increased measurably in both cases regardless of the nature of the cation. This suggested that calcium, although not as good as iron(III), is a suitable agent for modification and metal retention on the surface is the key component in designing tunable bead systems for sulfate uptake.
Relative number of articles concerned with sulfate, phosphate, nitrate Figure S1 shows the number of articles that address the adsorption-based removal of sulfate. Figure S1: Number of articles published between 2011 and early 2020 with the key words ʺsulfateʺ and ʺadsorptionʺ, ʺphosphateʺ and ʺadsorptionʺ and ʺnitrateʺ and ʺadsorptionʺ in the article title (source: Scopus).

X-Ray Photoelectron Spectroscopy
Surface characterization of the synthesized bead systems played was crucial in developing an understanding of the materials and XPS was used to analyze the effect of cross-linking and metal imbibing. The XPS analysis (wide scan) showed presence of calcium within the unmodified system, most likely impurities from the purchased chitosan used during synthesis as seen in Fig. S2. The narrow scan of the nitrogen region showed increased presence of a side band after modification rather modification independently of calcium imbibing suggesting no strong coordination effects of calcium ions (cf. Fig. S3). The narrow scan of calcium (cf. Fig. S4) on the other hand shows no peak shift itself suggesting no strong chelation or change in coordination. The narrow scan of carbon highlighted a unique separate band of the material with the lowest cross-linking ratio of 1:10 (283.85 eV) which more and more developed into a smaller shoulder peak of 282.2 eV with decreasing intensity. The same trend was observed with 285.4 eV with the exception of the material with a CL-ratio of 1:1 w/ Ca which showed a separate peak at 286.2 eV that is not observed for the other materials (cf. Fig. S5).    Analysis of the values obained via isotherm studies may allow for more insights into binding of adsorbate to adsorbent. The obtained isotherm values from the Sips istherm analysis are provided in Table S2. Table S2: Determination of Qe, k and n values of four selected, calcium imbibed bead systems.

Size Distribution
To evaluate the size distribution of the bead systems, the diameter of multiple beads (n=15) per system was measured via a digital caliper (see Fig. S8 and Table S3). Figure S8: Size of different bead systems from left to right: non-modified, 1:15 Cl-ratio, 1:10 Cl-ratio, 1:5 Cl-ratio, 1:1 Cl-ratio and 5:1 Cl-ratio.