Optimization of the preparation of thiourea-based compounds modifying PVA adsorbents

In order to improve the adsorption capacity of poly(vinyl alcohol) (PVA) for heavy metals, a new thiourea-modified poly(vinyl alcohol) adsorbent (TU-SPVA) was prepared using PVA as the raw material, glutaraldehyde as the cross-linking agent and thiourea (TU) as the modifier. The preparation conditions of TU-SPVA were optimized by one-way experiments and response surface methodology (CCD), and the adsorption mechanism was investigated using adsorption kinetics, adsorption isotherm, and adsorption thermodynamics, and the results showed that the optimum conditions for the preparation of TU-SPVA were as follows: pH 6.06, m(TU):m(SPVA) = 6.1:1, and the preparation time of 2.47 h. The maximum removal rate of Cr(VI) by TU-SPVA was 99.37%. The modification reaction in the preparation of TU-SPVA mainly occurred on the hydroxyl group (-OH) of the PVA molecular structure, and the -NH₂ and -C = S functional groups, which can be coordinated with heavy metal ions, were introduced through hydroxyl aldehyde condensation and Schiff reaction. The adsorption of Cr(VI) by TU-SPVA was more consistent with the quasi-secondary kinetic equation and Langmuir model, and the adsorption is a non-spontaneous exothermic process. Combined with the results of scanning electron microscopy and infrared spectroscopy, the adsorption mechanism of TU-SPVA on Cr(VI) is mainly coordination, ion exchange and electrostatic effect, and void filling.

Graphical abstract

The graft modification method was used to prepare TU-SPVA, and the optimum preparation conditions of TU-SPVA were investigated, TU-SPVA has a more obvious pore structure than PVA. The adsorption mechanism of TU-SPVA on Cr(VI) was analyzed using the Langmuir et al. adsorption model.