Ca(OH)2 coated nanoscale zero-valent iron as a persulfate activator for the degradation of sulfamethazine in aqueous solution

•nZVI-Ca(OH)2 was used as an activator of PS for the degradation of SMT.•There was an optimum dosage of nZVI-Ca(OH)2 for the activation of PS to degrade SMT.•nZVI-Ca(OH)2 was efficient for PS activation in synthetic groundwater at pH 3–9.•The presence of high concentration of HCO3− or SO42− greatly inhibited SMT removal.•The Ca(OH)2 shell can protect the inner iron core from oxidation in the air.