A fast, low-cost and scalable method was developed to prepare basic aluminum carbonate (Al(OH)CO₃) porous nanospheres via a microwave-assisted solvothermal process in 5 min. All chemicals used were low-cost and environmentally benign. These Al(OH)CO₃ nanospheres had an extremely large surface area (484 m² g⁻¹), abundant carbonate groups, and showed superb adsorption properties with maximum capacities of 170 mg g⁻¹ and 60 mg g⁻¹ for As(V) and Cr(VI) at pH 7, respectively. These values were obviously higher than that of many other nanomaterials. In addition, for practical applications, 5 mg Al(OH)CO₃ nanospheres could purify 6 kg As(V) contaminated water, reducing the As(V) concentration from 100 ppb to less than 10 ppb. A new adsorption mechanism involving ion exchange between carbonate groups and As(V) or Cr(VI) species was proposed and confirmed.