pH- and Temperature-Responsive Dual-Network Hydrogel for Controlled Adsorption - Desorption Cycling of Basic Blue 41

Abstract

The release of cationic textile dyes such as Basic Blue 41 (BB41) into natural waters is still a pressing environmental issue, and adsorbents that can be regenerated and reused several times remain in demand. In this work a dual-network (DN) hydrogel sensitive to both pH and temperature was synthesised and examined for the controlled uptake and release of BB41. The first network was prepared from acrylic acid and acrylamide and supplies carboxyl/carboxylate groups that bind the cationic dye, whereas a second poly(N-isopropylacrylamide) (PNIPAM) network was introduced to confer thermo-sensitivity around its lower critical solution temperature (LCST, ≈ 33 °C). FTIR, XRD and FESEM confirmed that the two networks coexist and that the freeze-dried gel has a porous, sponge-like interior. Batch experiments showed a strong pH dependence, the uptake being greatest in mildly alkaline solution where the carboxyl groups are largely deprotonated. Equilibrium was reached in roughly 90 min and the kinetic data obeyed the pseudo second-order equation, while the Langmuir model gave the best equilibrium fit with a maximum monolayer capacity of 384.6 mg g⁻¹. The thermodynamic functions pointed to a spontaneous, exothermic adsorption. Most importantly, lowering the pH to acidic values and warming the system above the LCST released the bound dye, allowing the gel to be reused for five consecutive cycles with only about 12 % loss of capacity. The results indicate that this stimuli-responsive DN hydrogel is a promising, reusable platform for the treatment of dye-bearing water.

Highlights
 A pH- and temperature-responsive dual-network hydrogel was prepared for BB41 capture.
 Carboxylate-bearing poly(AAc-co-AAm) and PNIPAM networks were combined in one matrix.
 Uptake followed pseudo-second-order kinetics and a Langmuir isotherm (qₘ ≈ 385 mg g⁻¹).
 Dye release was triggered by acidic pH together with heating above the LCST (≈ 33 °C).
 About 88 % of the initial capacity was retained after five adsorption–desorption cycles.