Developing superior, rapid, cost-effective adsorbents derived from organic spent adsorbent is an economically sustainable way for purifying azo dye wastewater. Herein, we report a precursor-calcination strategy for the recycle of the organic spent adsorbent to a high value-added three-dimensional sulfate-functionalized MgAl-layered double oxide (3S-LDO). Thanks to the unique property of the sulfate group and LDO, 3S-LDO exhibited a superior (4340.71 mg/g) and ultrafast (< 1 hour) adsorption toward methyl orange (MO, as the representative of azo dye). A thermodynamic study revealed that the reaction process was spontaneous and exothermic. FT-IR, XPS, and XRD results confirmed that the sulfate from 3S-LDO played a vital role in MO removal wherein the S = O bond (with the electrophilic character) from SO42− interacted with the N = N double bond (with rich electron) in MO through the electron donor-acceptor mechanism. And the “memory effect” and surface complexation of 3S-LDO further strengthened the MO adsorption. More importantly, 3S-LDO could work efficiently in a wide pH range and even in the presence of competitive anions (e.g., Cl−, NO3−, and CO32−). Multiple cyclic runs and selective tests demonstrated the excellent reusability and explicit selectivity of 3S-LDO. This work not only provides a prospective sulfate-functionalized adsorbent from organic waste for rapid azo dye removal from wastewater but also achieves the high value-added utilization of organic waste.