Efficient removal of molybdate from groundwater with visible color changes using wasted aerobic granular sludge

•Aerobic granular sludge (AGS) shows excellent Mo adsorption capacity of 190 mg/g.•The polluted groundwater with Mo level <500 μg/L can be remediated to safe range.•Polysaccharides secreted by AGS contributed more to Mo removal than proteins.•Precipitate formation of molybdate with cations resulted in color changes of AGS.•This study provides a promising material for Mo removal with possible visible endpoint.