Removal of hexavalent chromium and methylene blue using volcanic tuff as an adsorbent

The removal of toxic pollutants such as hexavalent chromium (Cr (VI)) and methylene blue (MB) from aqueous solutions is of significant concern due to their harmful environmental and health effects .This study examined the potential of volcanic tuff, a naturally occurring and reasonably priced material, as an adsorbent for the removal of MB and Cr (VI) from aqueous solutions. To optimize process parameters such as pH, contact time, adsorbent dosage, initial adsorbate concentration, and temperature, batch adsorption experiments were carried out using unmodified (Normal), acid-modified (AT), and base-modified (BT) tuff. At optimum conditions (pH=2, 20 ppm Cr (VI), 0.1g adsorbent, 25 oC, 60 min), the acid-modified tuff demonstrated the highest efficiency (%25.01) for Cr (VI) removal. The equilibrium data fit the Temkin isotherm model (R2=0.985), and the process kinetics were in accordance with the pseudo-second-order model (R2=0.994), suggesting chemisorption as the rate-limiting step. At 10 ppm MB concentration, pH=5, 0.2 g adsorbent, 25 oC, and 60 min, the base-modified tuff demonstrated nearly total removal (%100) and maximum efficiency for MB adsorption. The pseudo-second-order model (R2=0.9989) was also supported by the MB adsorption kinetics, and the Langmuir isotherm (R2=0.992) best described the equilibrium, indicating monolayer adsorption. Taguchi optimization verified that pH and initial concentration were the most important factors for Cr (VI) removal (%47.18 and %27.27 contribution, respectively), whereas temperature and initial concentration dominated MB removal (%47.74 and %33.46). SEM, XRD and FTIR analyses verified that chemical modification increased surface reactivity by changing the aluminosilicate structure. Volcanic tuff is a reliable and promising low-cost adsorbent for wastewater treatment applications involving both cationic dyes and anionic heavy metals.