<p> Supplementary information</p><p>Remazol red dye removal using Poly(acrylamide-co-acrylic acid) hydrogels and water absorbency studies</p><p>Miguel A. Corona-Rivera, Víctor M. Ovando-Medina, Luis A. Bernal-Jacome, Elsa Cervantes-González, Iveth D. Antonio-Carmona, Nancy E. Dávila-Guzmán</p><p>Zero point charge (ZPC) determination.</p><p>Zero point charge (ZPC) of hydrogels was determined as follows: 50 mL of distilled water were poured into a 100 mL Erlenmeyer flask, adjusting the initial pH between 2 and 11 by adding HCl or NaOH 0.1 N solutions. Afterwards, 0.5 g of dried hydrogel was added, and after 48 h of magnetic stirring at room temperature, the final pH was measured. The ZPC corresponds to the point where the curve of final pH as a function of initial pH crosses the diagonal at 45 °.</p><p>Figure S1. Zero point charge diagram corresponding to hydrogel 4-AAm/AAc 70/30.</p><p>Adsorption-Desorption cycles Dye adsorption experiments were made as described in the methodology of manuscript using 50, 100 and 150 mg/L of initial dye concentration. Desorptions were achieved by placing the saturated hydrogel with dye from adsorption experiment in 20 mL of NaOH 0.1 N aqueous solution through 5 days at room temperature, measuring the final dye concentration in the aqueous solutions. Desorption percentage were calculated from a mass balance. Results are shown in Figure S2. It was observed that for high initial dye concentrations (100 and 150 mg/L) some dye remains adsorbed onto hydrogel after desorption, resulting in lower dye adsorption after the first cycle. Figure S2. Adsorption-desorption cycles of RR3BS dye onto hydrogel 4-AAm/AAc 70/30 at 50, 100 and 150 mg/L of initial dye concentration.</p><p>SEM images of dried hydrogels. SEM analyses were made in a EDX, Philips XL 30 instrument (SEMTech solutions, North Billerica, MASEM). Images shown in Figure S3 and S4 were obtained for hydrogels from runs 4- AAm/AAc 70/30 and 9-AAm/AAc 70/30 with 1% and 2% of cross-linking agent, respectively.</p><p>Figure S3.SEM images of hydrogel corresponding to run 4-AAm/AAc 70/30.</p><p>Figure S4.SEM images of hydrogels corresponding to runs 4-AAm/AAc 70/30 and 9-AAm/AAc 70/30.</p>