Sulfamic acid functionalized slag for effective removal of organic dye and toxic metal from the aqueous samples

Surface functionalization of blast furnace slag with sulfamic acid (a zwitterion) was performed for the removal of Cr³⁺ and methylene blue dye (MB) from water samples. The slag functionalization process was optimized using Response Surface Methodology Design. Statistical analysis of the parameters that include the sulfamic acid amount (A), reaction time (B), and temperature (C) revealed that (A) increase had a negative effect on the adsorption of both pollutants by the zwitterion slag, whereas (B) and (C) increase presented a positive impact. At the optimum condition of 2 g sulfamic acid amount, 50 min reaction time and 37 C temperature, the prepared slag showed a removal efficiency of more than 90% for both Cr³⁺ and MB. Surface characterization by SEM/EDS, FTIR, XPS and surface area analyser, showed an improvement in surface properties and the incorporation of zwitterionic NH₂⁺ and S=O groups of sulfamic acid. Adsorption isotherm and kinetic studies conducted with the zwitterion slag showed the adsorption process was suited to Freundlich isotherm model and pseudo-second-order kinetic model. The thermodynamic study conducted revealed the spontaneity of the process based on the calculated negative ΔG (Gibb’s free energy) values. The prepared zwitterion slag offered easy regeneration with dilute HCl solution and showed a considerable removal (Cr³⁺: 65% and MB: 80%) for both pollutants even after 3 cycles of usage.