Synthesis of Magnetic Chitosan Hydrogel from Crab Shells as an Environmentally Friendly Adsorbent for Water Purification

Abstract

Water pollution is a serious problem in the world. Crab shells are abundant bio-wastes which can be harnessed to produce useful chitosan. In this project, chitosan was synthesized from chitin extracted from crab shells. The degree of deacetylation and molecular weight of chitosan synthesized were determined by Fourier transformation infrared spectroscopy (FTIR) and mass spectrophotometry, respectively. A novel method of magnetic chitosan hydrogel (MCH) synthesis was devised based on literature concepts, allowing MCH to be synthesized one-pot and using a greener procedure of chitosan synthesis. MCH was as effective as commercial activated carbon (AC) and chitosan in removing direct red and acid blue dye and outperformed both adsorbents in removing methyl orange. The maximum adsorption capacity of MCH on direct red and acid blue derived from the Langmuir isotherm were higher than that of both unconverted chitosan and commercial AC and also several other adsorbents reported in literature. The unique magnetic property of both MCH renders it reusable while sustaining more than 90% removal of acid blue even after five cycles, which outperforms unconverted chitosan by a great margin. MCH is thus a promising and environmentally friendly adsorbent which is able to remove dyes rapidly from water.

Appendices

Appendix 1: FTIR of Chitosan Synthesized

The percentage of deacetylation (DDA) of the chitosan synthesized was determined by the following formula:

$${\text{DDA}}\left( \% \right) = \left[ {1 - \left( {A_{1655} /A_{3450} } \right)/1.33} \right] \times 100$$

Using FTIR software, A₁₆₅₅ and A₃₄₅₀ were determined to be 2.88 and 3.69 respectively (Fig. 49.10). DDA(%) was hence calculated to be 41.3%.

Appendix 2: Langmuir and Freundlich Isotherm

The equilibrium concentration data obtained from initial concentration tests on acid blue and direct red were fitted into Langmuir isotherm and Freundlich isotherm. The Langmuir isotherm assumes that the adsorbed material (such as acid blue) is adsorbed over a uniform adsorbent surface at a constant temperature.

The linear form of Langmuir isotherm equation is given by:

$$\frac{{C}_{e}}{{q}_{e}}=\frac{1}{b{q}_{m}}+\frac{{C}_{e}}{{q}_{m}}$$

where ${C}_{e}$ is the equilibrium concentration of dye (mg/L), ${q}_{e}$ is the equilibrium capacity of the sorbents (mg/g), b is the Langmuir constant that indicates the sorption intensity, and q_m is the maximum sorption capacity (mg/g).

The Freundlich isotherm assumes that the adsorption occurs on a heterogeneous surface.

The linear form of Freundlich equation is given by:

$$\mathrm{log}\left({q}_{e}\right)=\mathrm{log}\left({K}_{F}\right)+\frac{1}{n}\mathrm{log}({C}_{e})$$

where ${C}_{e}$ is the equilibrium concentration of dye (mg/L), ${q}_{e}$ is the equilibrium capacity of the sorbents (mg/g), K_F, a constant, is related to sorption capacity and n corresponds to sorption intensity.

If the equilibrium concentration data fits the Langmuir isotherm, adsorption can be inferred to be monolayer. Maximum adsorption capacity of the MCH and chitosan can be derived from the inverse of the gradient of Langmuir linear equations.

In contrast, if the equilibrium concentration data fits the Freundlich isotherm, adsorption can be inferred to occur on a heterogeneous surface and adsorption is multilayer. A comparison between the R² values of the graph can determine which model the data set fits better (Table 49.2).

Table 49.2 R² value comparison of Langmuir and Freundlich isotherms for acid blue and direct red

All data sets had a better fit for the Langmuir isotherm, implying that both MCH and chitosan have a homogenous surface with monolayer adsorption.