Preparation of PVA hydrogel beads and adsorption mechanism for advanced phosphate removal

• PVA hydrogel with Al3+ on network structure for phosphate removal was prepared.
• The monolayer chemical adsorption mechanism of phosphate was confirmed.
• PVA hydrogel presented high phosphate adsorption capacity and good reusability.

Poly(vinyl alcohol) (PVA) hydrogel beads with aluminum ion on its cross-linking structure were prepared by chemical cross-linking in the mixed aqueous solution of saturated boric acid and AlCl3. It was found that with increasing AlCl3 concentration and cross-linking time, the permeability of PVA hydrogel decreased, and the phosphate adsorption capacity of the hydrogel presented the trend of first increasing and then decreasing. For the hydrogel after phosphate adsorption, a new peak at 1044 cm−1 attributed to the bending vibration of phosphate appeared in FTIR analysis, the surface became smooth, and the characteristic signal of phosphate also appeared in EDAX analysis, indicating that phosphate was adsorbed onto PVA hydrogel by a relatively strong bonding. Aluminum can be found in the hydrogel both before and after adsorption, indicating that aluminum ion has been grafted onto the PVA hydrogel beads. To study the mechanism of adsorption process, we examined the Lagergren pseudo-first-order kinetic model, and pseudo-second-order kinetic model, and the intra-particular diffusion model, and found pseudo-second-order kinetic model exhibited the best correlation with our experimental data. Furthermore, our adsorption equilibrium data could be better described by the Langmuir adsorption isotherm model, indicating the phosphate adsorbed on the PVA hydrogel beads was the homogeneous nature of the monolayer adsorption. The maximum phosphate adsorption capacity of the PVA hydrogel reached 11.5 mg-P/g at proper pH value and initial concentration of the phosphate solution. A relatively good reusability of the PVA hydrogel beads for phosphate removal can be achieved.